An industrial automation input output module comprising: an outer housing member comprising a top housing wall and an outer peripheral housing wall, an inner housing member, the inner housing member being adapted for nesting within the outer housing member and comprising a plurality of tabs that are received in corresponding recesses defined by the outer housing member so that the inner housing member and the outer housing member are mechanically interlocked with a close snap-fit; and a one-piece elastomeric main seal, characterized in that said top housing wall and outer peripheral housing wall define an open recess, said outer housing member further comprising an inner peripheral housing wall spaced from said outer peripheral housing wall, wherein a groove is defined between said inner peripheral housing wall and said outer peripheral housing wall; said inner housing member is received in said open recess of said outer housing member so that said inner and outer housing member together define an interior space for containing at least one circuit board, said inner housing member comprising a projecting seal engagement wall that is received in said groove; and said one-piece elastomeric main seal is installed adjacent said groove, said main seal comprising: a seal inner wall, a seal outer wall and a seal end wall defining a U-shaped wall receiving structure adapted for close sliding receipt of said inner peripheral housing wall of said outer housing member, whereby said seal outer wall, located adjacent said groove, comprises an outer face that provides a fluid-tight lateral seal portion between an inner face of said seal engagement wall and said inner peripheral housing wall; and said seal further comprises a face seal portion defined by a lip that projects transversely outward from said seal outer wall away from said seal inner wall at a location spaced from said seal end wall so as to be located in said groove, in abutment with an inner wall of said groove that extends between and interconnects said inner and outer peripheral walls (24, 22) of said outer housing member, thereby providing a fluid-tight seal between a tip of said seal engagement wall and said outer housing member. an outer housing member comprising a top housing wall and an outer peripheral housing wall, an inner housing member, the inner housing member being adapted for nesting within the outer housing member and comprising a plurality of tabs that are received in corresponding recesses defined by the outer housing member so that the inner housing member and the outer housing member are mechanically interlocked with a close snap-fit; and a one-piece elastomeric main seal, characterized in that said top housing wall and outer peripheral housing wall define an open recess, said outer housing member further comprising an inner peripheral housing wall spaced from said outer peripheral housing wall, wherein a groove is defined between said inner peripheral housing wall and said outer peripheral housing wall; said inner housing member is received in said open recess of said outer housing member so that said inner and outer housing member together define an interior space for containing at least one circuit board, said inner housing member comprising a projecting seal engagement wall that is received in said groove; and said one-piece elastomeric main seal is installed adjacent said groove, said main seal comprising: a seal inner wall, a seal outer wall and a seal end wall defining a U-shaped wall receiving structure adapted for close sliding receipt of said inner peripheral housing wall of said outer housing member, whereby said seal outer wall, located adjacent said groove, comprises an outer face that provides a fluid-tight lateral seal portion between an inner face of said seal engagement wall and said inner peripheral housing wall; and said seal further comprises a face seal portion defined by a lip that projects transversely outward from said seal outer wall away from said seal inner wall at a location spaced from said seal end wall so as to be located in said groove, in abutment with an inner wall of said groove that extends between and interconnects said inner and outer peripheral walls (24, 22) of said outer housing member, thereby providing a fluid-tight seal between a tip of said seal engagement wall and said outer housing member. a seal inner wall, a seal outer wall and a seal end wall defining a U-shaped wall receiving structure adapted for close sliding receipt of said inner peripheral housing wall of said outer housing member, whereby said seal outer wall, located adjacent said groove, comprises an outer face that provides a fluid-tight lateral seal portion between an inner face of said seal engagement wall and said inner peripheral housing wall; and said seal further comprises a face seal portion defined by a lip that projects transversely outward from said seal outer wall away from said seal inner wall at a location spaced from said seal end wall so as to be located in said groove, in abutment with an inner wall of said groove that extends between and interconnects said inner and outer peripheral walls (24, 22) of said outer housing member, thereby providing a fluid-tight seal between a tip of said seal engagement wall and said outer housing member.