Sperm competition occurs when a female remates before her initial sperm supply is exhausted. Sperm remained viable in a female Onymacris unguicularis (Haag) (Tenebrionidae: Coleoptera) for 7 weeks, but laboratory observations showed a mean number of 5 days between matings. An anatomical basis for sperm displacement was indicated whereby the previous spermatophore was ejected from the female during a subsequent mating. Using the sterile-male technique, there was shown to be a large measure of sperm precedence from the last male to mate with the female. Keywords: Sperm displacement, sperm precedence, sperm viability, mating frequency, irradiation sterilisation, tenebrionid.