Dinobryon spp. are common components of the phytoplankton in temperate lakes. While these chrysophytes link bacterial carbon as well as primary production to the larger food web, few studies have examined their vertical distribution over a season. Data were collected over a 2.5 yr period in mesotrophic Lake Lacawac in eastern Pennsylvania (USA) to examine water column attributes relating to seasonality of the mixotrophic alga D. cylindricum, and specifically to address a hypo thesis that abundance and vertical distribution in the genus are associated with temperature. This information was used to guide laboratory experiments examining temperature and light effects on growth, feeding, and vertical migration of cultured Dinobryon. Data from Lake Lacawac and the literature indicated that high abundances of Dinobryon were associated with a narrow range of temperature (9−18°C) relative to temperatures where they were present (3−26°C). High abundances of several species of Dynobryon were associated with this temperature range, and occurred from late winter to late spring, depending on latitude and depth in the water column. Although high Dinobryon abundances were not tied directly to levels of photosynthetically active radiation in Lake Lacawac, a UV-exclusion experiment in a nearby oligotrophic lake indicated a temperature-dependent negative effect of UV radiation. UV may limit the occurrence of Dinobryon populations in surface waters of clear oligotrophic lakes. Laboratory experiments showed that both light and temperature affected growth and feeding rates of Dinobryon, and that maximum feeding and growth rates occurred within the temperature range where high Dinobryon abundances were observed in field studies.