Previous studies on interactions between Sea Lamprey and Lake Trout have focused on direct mortality of Lake Trout populations resulting from Sea Lamprey parasitism. However, an estimated 45-75% of Lake Trout survive a parasitism event. Little is known about how the reproductive physiology of survivors is affected or what the consequences are for Lake Trout populations. Previous studies have indicated Sea Lamprey parasitism affects Lake Trout reproduction in the short term by suppressing plasma sex-steroid concentrations as well as other reproductive endpoints. This study focuses on sublethal effects of Sea Lamprey parasitism on long-term testosterone production and milt concentrations in male lean and siscowet Lake Trout. Plasma and milt samples were collected from both morphotypes of parasitized and non-parasitized Lake Trout one year following sea lamprey parasitism. Milt and testosterone concentrations were compared from parasitized and unparasitized individuals. Parasitized siscowet Lake Trout had significantly lower milt concentrations one year after parasitism than their unparasitized counterparts. Lean lake trout displayed the same trend, but differences were more subtle. Conversely, for both morphotypes, parasitized individuals had higher plasma testosterone concentrations than unparasitized individuals, albeit not significantly. These results provide insight into the mechanism driving reproductive disruption in parasitized Lake Trout and will be useful for predicting effects at the population level.