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The mating patterns of a Namib tenebrionid (Onymacris bicolor) was studied in the field and laboratory. Females mated several times, while males guarded the females after copulation. Paired females were always larger than single ones. Paired and single males that were active above the sand did not differ in size. However, paired males above the sand were smaller and showed greater variation than paired, buried males. Size-assortative mating was weak above the sand, but male and female size corresponded better in buried pairs. Female fecundity increased with size. Eggs were laid at irregular intervals. Males preferred to mate with females that carried relatively more eggs. In general, males chose large mates, guarded large females for longer and were less likely to leave large females than small females when encountered by another female shortly after copulation. Females showed no preference towards large mates. When two males were present simultaneously, the first to meet a female was the one to mate with her. Nevertheless, male combat was observed frequently and take-overs occurred. Male size was important when a male was challenged by a rival shortly after copulation. Large challengers were equally likely to challenge for large or small females, but small males were more likely to challenge for small females. Moreover, with an increasing ratio of males to females the proportion of size-assorted mating pairs also increased. Mating pattern in O. bicolor was due to preference for large mates, scramble competition, and inter-male combat combined with the advantage of large size. In fighting, large size increased a male’s change of mating when inter-male competition increased. The weak degree of size-assortative mating in the field was probably due to low inter-male competition.