Mating is generally assumed to be costly, but mating costs differ between the sexes. Although mating itself is considered cheaper for males, mate search and mate competition are cheaper for females. Nevertheless, studies increasingly reveal considerable mating costs for males, and these costs should depend on the body size of the individual. We investigated size-dependent predation (ecological) and energetic (physiological) mating costs in male black scavenger flies, Sepsis cynipsea (Diptera: Sepsidae), a model organism for studies of reproductive behavior. We addressed costs of mating by assessing predation risk for differently sized flies in male, female, and mixed-sex groups. Males were not more likely to be predated in mating or mate-search situations. Scathophaga stercoraria (Diptera: Scathophagidae) predators preferred smaller females and males as prey. Male movement in these different social situations does not proximately explain this size-selective predation, as small individuals were not more mobile. We addressed energetic costs of mating by measuring residual longevity (or starvation resistance) of starved males exposed to different mating situations. Copulation, courtship, interaction with reluctant females, or brief interactions with other males, all presumably increasing energy demand, did not significantly reduce longevity of males compared with males not interacting with other individuals. In general, small males died sooner when starved. Overall, we found no direct costs of mating for male S. cynipsea, but both predation and physiological costs were size dependent.