This study characterized the seasonal variation of the steroid hormones testosterone (T), 11-ketotestosterone (11-KT), 17beta-estradiol (E2), and cortisol (F) as they relate to the gonadal development and reproductive behavior of the plainfin midshipman fish, Porichthys notatus. The plainfin midshipman is a deep-water teleost that seasonally migrates into the shallow intertidal zone where type I, or "singing," males build nests, acoustically court and spawn with females. The gonadosomatic index and plasma steroid levels were measured from adult type I males and females collected over four time periods (non-reproductive, pre-nesting, nesting, and post-nesting) that corresponded to seasonal fluctuations in midshipman reproductive biology and behavior. Among type I males, plasma levels of T and 11-KT were low during the winter non-reproductive period, gradually increased during seasonal recrudescence of the testes in the spring pre-nesting period, and then peaked at the beginning of the summer nesting period. In the latter half of the nesting period and during the fall post-nesting period, plasma levels of T and 11-KT were low or non-detectable. Low, detectable levels of E2 were also found in the plasma of 50% or more type I males during every seasonal period except the winter non-reproductive period. Among females, plasma levels of T and E2 were low throughout the year but briefly peaked in April during the spring pre-nesting period when ovaries underwent seasonal recrudescence. Plasma F levels were correlated with collection depth and were lower in males than females when fish were collected deeper than 120 m. The sex-specific peaks of steroid hormone levels in male and female midshipman may serve differential functions related to the physiology, reproductive behavior, and vocal communication of this species.