The primary goals of this study were to evaluate diurnal patterns of and sex differences in the levels of cortisol, 11-ketotestosterone, testosterone, and 17beta-estradiol in the sex-changing bluebanded goby Lythrypnus dalli. Steroid hormones were collected from water samples and analyzed by enzyme immunoassay. During the breeding season, hormones were sampled from both males and females at seven time points between 0600 and 2000 h. When comparing each time point separately, there were significant overall time effects for cortisol and 17beta-estradiol. Cortisol concentrations were lowest at the 0800-1000 h sampling point and showed a qualitative peak in late morning (1000-1200 h). Concentrations of 17beta-estradiol were elevated at the last sampling point (1800-2000 h). Broader temporal trends were revealed for testosterone and 11-ketotestosterone concentrations, both of which were elevated in the morning. There were no sex differences in overall hormone concentrations or temporal profiles for cortisol, 11-ketotestosterone, or testosterone. Males and females showed similar diurnal patterns of 17beta-estradiol but females had significantly higher water-borne 17beta-estradiol levels than males. The results show the presence of diurnal changes in steroid hormone levels in male and female bluebanded gobies. The lack of sex differences in androgens suggests that males of this species, and perhaps other bi-directional sex-changing species in which males do not exhibit prominent secondary sexual characteristics, do not require persistent elevations in 11-ketotestosterone or testosterone to maintain the male phenotype. Although the role of 17beta-estradiol in maintaining sex differences in sexually plastic species is unclear, our results suggest that, of the hormones measured, 17beta-estradiol has the greatest potential for future studies interested in this question.