Mysis diluviana is a small shrimplike crustacean native to the Laurentian Great Lakes. They are vital to maintaining the ecosystem of the Great Lakes due to their mid-level trophic status and diel vertical migration behavior that links benthic and pelagic habitat. It has been assumed that the entire mysid population performs a complete diel vertical migration, but recent evidence has suggested that some mysid populations exhibit a partial diel vertical migration (PDVM). This consists of some mysids remaining in the benthos at night, and others in the water column during the day. This study examined whole water column mysid net tows along a depth gradient in Lake Ontario to compare daytime and nighttime catches in order to determine if mysids exhibit PDVM in Lake Ontario, and answer whether depth and life stage influence the extent of PDVM. Very few mysids were caught during the day below 175m, but at 175m and 200m, an average of 276 and 476 mysids were caught per net tow, respectively. At 200m this daytime catch represented 87.5% of the nighttime catch. In addition, the vast majority of those mysids caught during the day at each site were juveniles. However, at 200m, there was very little difference between mysid day and nighttime catches and length distributions. With these results, it can be concluded that mysids do exhibit PDVM in Lake Ontario by not fully retreating to the benthos during the day. Juvenile individuals are also more likely to exhibit PDVM than adults, and mysids in general are more likely to exhibit PDVM at deeper sites. There are many possible explanations for why juvenile mysids are more likely to exhibit PDVM, and it is likely that low light levels in the water column at deeper sites contributes to this behavior. More research on mysid PDVM is necessary to understand their nuanced behavior and gain insight into how traditional mysid sampling, which is currently only done in the water column at night, may be biased.