An aerodynamic trimmer head assembly for use in flexible line rotary trimmers that provides reduced drag on the line and cooling air flow for the trimmer head drive mechanism during use, said assembly comprising: a trimmer head,,,,, including a housing,, adapted to be operatively connected to a rotary trimmer to effect rotation of the head, said housing defining at least one opening,, therein for the passage of a portion (17") of a length of line therethrough and having a generally curvilinear upper trimmer head surface,,,; characterised in that the assembly further comprises a shroud, operatively connected to said trimmer head,, for rotation with said trimmer head, said shroud defining an annular generally curvilinear portion disposed above and extending along said generally curvilinear upper trimmer head surface,,,; and a plurality of laterally spaced ribs, projecting from said shroud, or said upper trimmer head surface,,,, said ribs extending downwardly and forwardly in the direction of rotation between said generally curvilinear portions of said shroud and said upper trimmer head surface,,, along curvilinear paths from an upper portion of said shroud to a lower portion thereof so as to define a plurality of air channels, extending downwardly and forwardly between portions of said upper head surface and said shroud along said paths defined by said ribs, said channels defining a plurality of channel inlet openings disposed about an upper central portion of said shroud, such that during rotation of said head and shroud, air is drawn downwardly through said channel inlet openings, into and through said channels, and outwardly therefrom at a forward inclination about the length of line projecting from the housing thereby reducing aerodynamic drag on the line and cooling the drive mechanism of the rotary trimmer. a trimmer head,,,,, including a housing,, adapted to be operatively connected to a rotary trimmer to effect rotation of the head, said housing defining at least one opening,, therein for the passage of a portion (17") of a length of line therethrough and having a generally curvilinear upper trimmer head surface,,,; characterised in that the assembly further comprises a shroud, operatively connected to said trimmer head,, for rotation with said trimmer head, said shroud defining an annular generally curvilinear portion disposed above and extending along said generally curvilinear upper trimmer head surface,,,; and a plurality of laterally spaced ribs, projecting from said shroud, or said upper trimmer head surface,,,, said ribs extending downwardly and forwardly in the direction of rotation between said generally curvilinear portions of said shroud and said upper trimmer head surface,,, along curvilinear paths from an upper portion of said shroud to a lower portion thereof so as to define a plurality of air channels, extending downwardly and forwardly between portions of said upper head surface and said shroud along said paths defined by said ribs, said channels defining a plurality of channel inlet openings disposed about an upper central portion of said shroud, such that during rotation of said head and shroud, air is drawn downwardly through said channel inlet openings, into and through said channels, and outwardly therefrom at a forward inclination about the length of line projecting from the housing thereby reducing aerodynamic drag on the line and cooling the drive mechanism of the rotary trimmer.