A quantitative method is described for distinguishing two types of mouse sperm motility using a new parameter, the Linear Index. One hundred spermatozoa from C57BL/6-+/+ and 100 spermatozoa from C57BL/6-tw32/ + mice were used to evaluate this parameter. Spermatozoa were tracked by videomicrography, and the following calculated: net displacement velocity, Vn; curvilinear velocity of the sperm head, Vc; the "average" velocity, Va (a 5-point moving average of the track); the progressiveness ratio, (Vn/Vc); the curvilinear progressiveness ratio, (Va/Vc); and a linear index, (Vn/Va). Frequency histograms of linear index values were bimodal for both groups and each had a clear antimode at 0.5, separating two discrete subpopulations of spermatozoa. Within each group, "linear" (greater than 0.5) and "nonlinear" (less than 0.5) tracks differed in all characteristics; between groups, linear spermatozoa were similar and nonlinear spermatozoa differed only in the curvilinear velocity of the sperm head.