We studied diel variability in vertical distribution, feeding behavior and grazing impact of female Temora longicornis in Long Island Sound on seven cruises from March to July. T. longicornis usually performed diel vertical migration characterized by deep residence during the day and ascent to near-surface waters at night for variable periods. The pattern of diel migration was independent of either the vertical distribution or relative abundance of chlorophyll in the water column. There was no clear evidence linking the amplitude of vertical migration to food concentration. Rather, the amplitude of migration decreased toward the end of the season probably due to animals avoiding warm waters (> 17°C) near the surface. Gut pigment content showed diel variation characterized by maximum values during the nighttime. However, the estimated mean ingestion rate from the nighttime period was significantly greater than that of the daytime period in only 2 of 11 comparisons indicating that this copepod usually fed throughout the day at about the same rate. The shape of the diel curve was usually similar for females at 5 and 20 m. Usually there was no difference in gut content of females with depth even when differences in chlorophyll with depth were pronounced. Therefore, the diel variability in gut content was unlikely to result from continuous feeding in a vertically stratified food environment. Short-term (hourly) changes in chlorophyll concentration could not entirely account for changes in gut content over a diel cycle. We estimate that female T. longicornis removed daily < 1–34% of the phytoplankton stock and < 1–49% of the primary production in Long Island Sound. Estimates of daily carbon rations indicate that a herbivorous diet can satisfy the metabolic requirements and support egg production of T. longicornis throughout most of its season.