Mounting evidence shows that males adaptively alter their sperm quality, in addition to sperm quantity, in response to their perceived social environment and risk of sperm competition. Generally, subordinate males, facing a greater likelihood of sperm competition, produce faster or more motile sperm – presumably to make the most of rare mating opportunities when they arise. Similarly, we would expect males that have been denied access to females to invest more in mating at the first opportunity. We tested this idea in the neriid fly Telostylinus angusticollis, and found that a large proportion of males that had been housed in isolation did not transfer any motile sperm during their first copulation. Moreover, females were reluctant to mate with these isolated males, kicking their legs to prevent males from mounting and mating. In contrast, males that were housed in male-only group cages transferred large amounts of motile sperm in their first copulation, and showed similar mating success to males housed in mixed-sex group cages. The apparent infertility of isolated males was quickly reversed, with males able to produce viable offspring within 24 hours of female introduction. We speculate that maintaining viable sperm is costly for males, and therefore males do not invest in reproductive readiness when they receive no social cues indicating the presence of conspecific individuals. The presence of other males appears to be a sufficient social cue to induce reproductive investment. Effects of male social environment on female fecundity and offspring quality will be examined to investigate whether this plasticity in reproductive investment has adaptive consequences for male reproductive success.