Abstract Inbreeding frequently leads to inbreeding depression, a general reduction in trait values and loss of fitness, and it appears that some sexually selected traits are especially sensitive to inbreeding, but sperm may be an exception. Additionally, because inbreeding depression is always in the direction of low fitness, it can reveal the direction of past selection acting on trait values. Here, we experimentally manipulate levels of inbreeding in a beetle (Tribolium castaneum) by full-sib mating for six generations. This breeding design allowed us to track the effects of increasing homozygosity on male reproductive traits (sperm and testes size), male size and lifespan, and reproductive output within inbred families, and on the heritability of these traits. All traits measured showed significant inbreeding depression and heritabilities tended to increase with inbreeding. Since inbreeding resulted in shorter sperm and smaller testes, it suggests that longer sperm and larger testes confer higher fitness in this beetle.