The smooth bay shrimp, Lissocrangon stylirostris, was sampled using a beach seine and a hyperbenthic sledge in the summer of 2006 at a dissipative sandy beach in southern Oregon. We investigated possible variations in the population structure due to environmental variables, preferred prey, sexual system employed, impacts due to parasitism by the bopyrid isopod Argeia pugettensis, and possible predators. The population was 63% female, and females were generally larger than males. The density of L. stylirostris varied significantly with the time of day. A cohort of small shrimp was observed from July to September while large adult shrimps were rare during the same months. The net effect was that shrimp densities were stable over the summer. The shrimps fed mostly at night on the mysid Archaeomysis grebnitzkii. The differences in female to male ratio, the larger size of females relative to males, and the relatively little overlap in size frequency distribution between males and females could be because this species exhibits a type of protandry. Both male and female shrimps supported an equal, high prevalence of, level of infestation by A. pugettensis (62%). The parasitized shrimps were larger in size than normal shrimps but had lower weight per length ratio. Very few brooding shrimps were parasitized (0.2%), and a higher percentage of parasitized shrimps had empty stomachs and significantly lower stomach contents. We hypothesize that the parasite is affecting the shrimp by producing variation in its normal size and weight, sterilizing females, and reducing its ability to capture food. Finally, individuals of English sole and Pacific staghorn sculpin individuals preyed on the shrimps.