In this study we compared the insemination efficiency of two alternative mating tactics (courtship and sneak mating) in the guppy Poecilia reticulata by quantifying the number of sperm delivered during a copulation. During a single copulation, guppies delivered between zero and 92% of the sperm available, as determined by mechanically stripping the male's sperm reserve at rest. The absolute number of sperm delivered after courtship was three times larger than that delivered through sneak mating; nonetheless, the variance was large with both tactics and the two distributions largely overlapped. The number of sperm available at rest increased with male size. With both tactics, the number of sperm delivered was positively correlated with the sperm available. Contrary to courtship copulations, in sneak copulations there was no correlation between the number of sperm delivered and male size. However, once the data were standardized for sperm reserve, small males delivered a larger proportion of their available sperm during sneak copulation. The rate of sexual acts (sigmoid and thrust rate) before copulation was not correlated with the number of sperm available. After the occurrence of a copulation in both the courtship and sneak copulation groups, the sexual activity of the male decreased in proportion to the amount of sperm he previously inseminated.