A circular knitting machine, comprising a footing that supports a needle cylinder that can be rotationally actuated, with respect to said footing about its own axis, which is orientated substantially vertically; a plurality of axial slots being formed on the outer lateral surface of the needle cylinder, each slot accommodating a needle; means (7,8,9,10) for actuating the needles being provided and interacting with said needles during the rotation of the needle cylinder about its own axis in order to actuate the needles along the corresponding axial slot of the needle cylinder so that the needles form knitting with at least one yarn dispensed to the needles at at least one drop or feed of the machine, said needle actuation means (7, 8, 9, 10) comprising, for each needle, a sub-needle arranged in the corresponding axial slot of the needle cylinder below the corresponding needle; said sub-needle being connected bilaterally to the corresponding needle in its motion along the corresponding axial slot and having, along its extension, a heel that is orientated radially with respect to the needle cylinder; said sub-needle being able to oscillate on a radial plane of the needle cylinder in order to pass from an active position, in which the sub-needle is extracted radially with its heel from the corresponding axial slot of the needle cylinder so as to engage paths formed by sub-needle actuation cams, which are arranged around the needle cylinder and are suitable to produce or allow a movement of said sub-needle along the corresponding axial slot of the needle cylinder, to an inactive position, in which it is embedded with its heel in the corresponding axial slot of the needle cylinder so as to avoid engaging said sub-needle actuation cams; sub-needle actuation means being provided which act on said sub-needle for its transition from said active position to said inactive position and vice versa; said means for actuating the sub-needle comprise an actuation element that is arranged in each one of said axial slots below the corresponding sub-needle and can move on command along the corresponding axial slot in order to interact with the lower end of the sub-needle and produce the transition and retention of said sub-needle in said active position or in said inactive position, characterized in that the lower end of said sub-needle is shaped complementarily to the upper end of said actuation element in order to move said sub-needle from said inactive position to said active position or vice versa as a consequence of an axial movement of said actuation element along the corresponding axial slot of the needle cylinder and in that said lower end of said sub-needle is forked, with a first prong that protrudes downward with respect to the second prong; said first prong lying closer to the bottom of the corresponding axial slot of the needle cylinder than said second prong; said first prong being engageable by an upper portion of said actuation element to move or stably retain said sub-needle in its inactive position, and said second prong being engageable by said upper portion of said actuation element to move or stably retain said sub-needle in the active position.