Theoretical and empirical studies of reproductive success and life-history evolution primarily focus on the regulation of female performance. This emphasis is justifiable only if it can be assumed that the determinants of male reproductive success are like those regulating female reproductive success. Yet, in virtually all multicellular organisms, there is substantial gender-specific variation in reproductive behavior, which leads to fundamental differences in the determinants of male and female reproductive success. One of the simplest and most phyletically widespread gender-specific differences in reproductive behavior is the retention of female gametes within the parent until some time after fertilization, and the broadcasting (or transfer by pollinators) of male gametes into air or water. This reproductive dichotomy potentially enhances levels of (1) intermale gametic competition, (2) gender-specific variance in pre-fertilization reproductive success, and (3) diversity of paternity within sibling relationships. Consequently, this sort of gender-specific variation in reproductive pattern potentially affects the intensity of sexual selection levels of gene flow, the structure of breeding systems, and the evolution of social behaviors. At present, virtually nothing is known about the determinants of male versus female reproductive success for any non-motile, aquatic invertebrate. This research will make direct field measurements of individual male (and, necessarily, female) reproductive success and dispersal in the colonial hydrozoan, Hydractinia milleri. DNA sequence analysis (i.e., DNA fingerprinting) to determine parent-offspring relationships of brooded larvae and adults that are members of, and recruit into, mapped populations over a three-year period. Data obtained from a combination of field and laboratory manipulations will allow an assessment of how colony density, size and proximity affect the relationship among reproductive investment, fertilization success, and reproductive success.