Reports the results of a study of mate preferences of the crested auklet Aethia cristatella, a monogamous seabird in which both sexes are ornamented. Variation in crest length among 253 adult crested auklets measured at Buldir Island, Alaska; Responses to male and female ornament manipulations; Indication that crested auklet ornament size is likely to be favored by sexual selection involving mutual mate choice by both sexes.