Abstract Mysids of the genus Neomysis are omnivorous, feeding on microalgae, zooplankton, and detritus, and are preyed upon by fish. Neomysis mirabilis is the most dominant mysid in the subarctic Notoro Lagoon, located on the coast of northeastern Hokkaido, Japan. The feeding habit of N. mirabilis was demonstrated as herbivorous during the icy season but has not yet been determined for the longer duration of the warm season. In this study, N. mirabilis were collected from the surface water of Notoro Lagoon in August to October 2023. To determine grazing by N. mirabilis on microalgae during the warm season we estimated the daily ration using gut pigment contents and the gut passage time in a gut evacuation experiment and then compared the data with the estimated carbon requirement. The average daily ration (carbon derived from the consumption of microalgae) ranged from 43.7 to 89.3 μg C ind.-1 day-1 from August to October. The percent composition of the daily ration of microalgae to the metabolic requirement of N. mirabilis ranged from 24.7% to 54.7%. These results reveal that the feeding habit of N. mirabilis can be omnivorous during the warm season.