Population comparisons yield important insights into adaptive differentiation. However, despite the current interest in sperm competition and spermatogenesis, geographic variation in these traits has received little attention. We tested the hypothesis that sperm production covaries with risk by comparing five natural populations of Trinidadian guppies (Poecilia reticulata): two that inhabited dangerous Crenicichla localities, two from low–risk Rivulus sites and a ‘transplant’ population comprised of the descendants of guppies moved from one of the high–risk sites to a low–predation environment. As predicted, males from the three low–risk sites performed significantly more courtship displays and had larger sperm reserves than their high–risk counterparts. This result implies higher rates of sperm competition in the low–risk sites.