ix, 55 p. : ill., maps. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Aggressive behaviors in many animals increase juvenile survival and reproductive success. I explored the role of aggressive behavior in maternal care and male-male competition in caprellid amphipods. Attacks of females in three reproductive states toward male and female conspecific and congeneric caprellids were quantified. Parental females showed greatest aggression toward males of both species, suggesting these caprellids may pose threats to juveniles. Paired males were exposed to females in three reproductive states and other males and fought most often in the presence of nonbrooding females. This finding suggests that males recognize and fight over receptive females. Males fight with the second gnathopods and "poison tooth," which may cause fatalities in conflicts. Mortality rates of males with and without (removed) poison teeth were not significantly different, but the highest observed mortality rate was no-tooth caprellids when paired with toothed males, suggesting presence of the teeth may impact the outcome of male-male conflicts.