There is increasing evidence that, under laboratory conditions, trypanosome populations which have the same genetic constitution by nature of the fact that they were derived from a cloned population but differ phenotypically in their expression of different surface variable antigens also differ in their infectivity and growth characteristics in certain environmental conditions. We have reviewed the evidence available for the existence of such a phenomenon in T. vivax and T. brucei variant populations and conclude that, in view of the fact that no physiological function has yet been attributed to the surface variable glycoprotein antigen, such differences most likely indicate concomitant expression of other genes controlling physiological processes which determine the parasite's ability to multiply in a given environment. The question of whether such a linkage between expression of a given surface variable antigen and expression of other physiological control genes is stable over extended periods of time or exists in all members of a trypanosome population displaying a given variable antigen is also raised but the experimental data at present available is too limited to allow any conclusions to be drawn.