The reproductive biology of Enallagma hageni was studied to determine the potential for sexual selection on this non-territorial, short-lived, promiscuous damselfly. Variance in male and female reproductive success, measured by variation in mating success and number of eggs fertilized over an individual’s lifetime, was quantified in an isolated population of individually marked Enallagma. Of the males, 41% failed to mate in their lifetime, compared to 3% mating failure by females. Lifetime mating success was positively correlated with longevity; effects of size, relative age, and female color morph were not significant. Dissection and sterile male experiments measuring sperm competition revealed that second mates can displace 83% of the first mate’s sperm by volume, and fertilize 96% of the female’s immediate clutch. Males guarded mates while they oviposited underwater. Non-contact guarding benefitted males by preventing displacement of sperm should thier mates prematurely resurface; females benefitted because guarding males saved them from drowning. Sperm competition benefits a female because it increases a male’s investment in her clutch, making him less likely to abandon her while she is submerged. Alternative mate-finding tactics were identified: males 1) searched for females around the pond or 2) waited at oviposition sites for resurfacing, gravid females. Although fitness gains from the searching tactic were greater than those accrued by the waiting tactic, the latter is maintained in the population because males using both tactics had a greater lifetime fitness than males using only the searching tactic. Total variance in male reproductive success was partititioned into variance due to differences in female fertility and variance due to differences in the number of mates per male. In E. hageni, 68-73% of the variance in number of mates per male resulted from variance in longevity and 27-32% resulted from differences in mating rates among males. Because mating success correlated positively with longevity, and differences in longevity may arise from random mortality, the potential for sexual selection in this species may be low, even if variation in mating rates is assumed to reflect gene differences among males.