Our understanding of littoral microcrustacea is limited in comparison to knowledge of zooplankton communities. Sida crystallina (Crustacea: Cladocera) is a filter feeding microcrustacean common to freshwater invertebrate communities in much of the United States and Europe. Although free-swimming forms of the species have been described, the more common "pond" form attaches to aquatic vascular plants by means of a cervical gland on the back of its carapace. Sida is thus confined principally to the littoral zone, and the effects of littoral conditions can be examined by comparing its population ecology to those of limnetic species. The behavior and population dynamics of Sida were studied in Cochran Lake, Michigan. Individuals frequently changed their position of attachment, and visually located new substrates. Frequent movement and spatially random swimming, particularly by juveniles, were sufficient to generate high densities of individuals on isolated or "edge" plants in a stand of vegetation. The white water lily Nymphaea odorata grows in numerous isolated stands in the lake. Population densities of Sida in these habitat islands were obtained by sampling leaves of Nymphaea, then estimating numbers of plant-associated organisms per m2 of lake surface from measures of leaf surface area and leaf density. Population dynamics of Sida were recorded at three locations in the lake at 1 to 2 week intervals from May to October, 1976. Densities were highest during June, then declined rapidly at all three locations. Sida persisted in low numbers during the remainder of the summer. The production of males and resting eggs occurred in October. An intensive analysis of birth, death and population growth rates was undertaken during June and July 1979. Birth rates were influenced by the proportion of adults in the population, the number of eggs produced per clutch, and the frequency with which new clutches of eggs were produced. The number of eggs per clutch varied with body size of parent. When body size was standardized, declines in clutch size were correlated with declining phytoplankton biomass and a shift in the phytoplankton community from diatoms and small flagellates to colonial green and blue-green algae. Populations of Sida kept in small tubular enclosures in the open water showed significantly higher rates of increase than did populations confined to enclosures placed among the Nymphaea leaves in the littoral zone. The presence of high densities of aquatic macrophytes therefore may also contribute to the decrease in productivity of Sida during the summer months. The Numphaea leaves did not alter the species composition or the primary production of the phytoplankton community in the littoral zone during June and July, however. Numbers of invertebrate predators capable of feeding upon Sida remained very low during the two months. The presence of largemouth bass fry in the littoral zone during late June coincided with estimates of high mortality, particularly of adults. Laboratory experiments confirmed the ability of the fry to forage from plant surfaces and documented their preference for large prey, particularly Sida. Field experiments, using six 4 m2 exclosures to which varying numbers of fry were added, further demonstrated that depletion of large plant-associated invertebrates like Sida may be a general feature of the foraging of fish fry in the littoral zone of lakes.