Additive genetic variances were estimated for several life history traits of north temperate Michigan and subtropical Florida populations of the freshwater copepod Mesocyclops edax (Crustacea: Copepoda). Estimates were obtained by rearing animals at 25 deg. C and using a paternal half sib design. Considerable additive genetic variance was revealed in female and male body sizes of the Florida population and in female and male maturation times in the Michigan population. Female maturation time and clutch size in the Florida population and female body size and clutch size in the Michigan population exhibited low additive genetic variance. These findings provide further evidence of substantial additive genetic variance in life history traits of natural populations. The contrasting trends in additive genetic variance underlying life history traits of these populations are interpreted as reflecting differences in the potential to respond to selection and possibly also the different selective regimes of the Michigan and Florida environments.