Theoretical models predict that individual males will increase their investment in ejaculates when there is a risk of sperm competition. Because the production of ejaculates is assumed to be energetically costly, only those males in good physical condition should be capable of producing ejaculates of high quality. We studied ejaculate investment (relative testis size, controlling for body size) as well as the size, behaviour, and energetics of spermatozoa in lake whitefish (Coregonus clupeaformis), a species in which males provide nothing but sperm to females during reproduction. Ejaculate investment was condition dependent, with males in better body condition having relatively larger testes. Sperm adenosine triphosphate (ATP) levels were unaffected by male condition alone, but increased with testis size when controlling statistically for both male size or body condition. Thus, males investing in relatively larger testes produced spermatozoa with higher energetic capacity. We also discovered testis asymmetry in this species, a phenomenon not previously reported in fishes, though widespread in other taxa. The mass of the (larger) left testis was a better predictor of sperm ATP stores than the mass of the (smaller) right testis (controlling for body size), suggesting that testis size asymmetry might be related to the production of high-quality sperm.