Histological and ultrastructural studies describing early life history stages of freshwater unionid bivalves and studies evaluating impacts of chemical contaminants on early growth are lacking. This study examined larval and juvenile shell ultrastructure of Lampsilis ventricosa using scanning electron microscopy (with observations of Pleurobema cordatum), described morphology of internal organs of laboratory-reared L. ventricosa and evaluated effects of cadmium on juvenile survival, growth and development. Postmetamorphic (juvenile) shell was added to peripheral edges of the larval shell. Initial juvenile shell growth was rapid and coincided with shell deposition into larval shell pores. Glochidia metamorphosed while encapsulated on gills of fish hosts. Presumptive digestive organs, gills, mantle, foot, and nervous system were present prior to excystment. Digestive glands were filled with lipid bodies. At release the stomach had differentiated into an anterior gastric portion and a posterior style sac. Subsequent developments in the alimentary tract consisted of intestinal elongation, further elaboration of ciliary sorting areas and ciliary linings, and ventral shift in alignment of the style sac. After 2 weeks, digestive glands were devoid of lipid bodies. Diverticulation of digestive glands began at 3 weeks. Gill filaments were added posteriorly, by papillary outgrowths, beginning at approximately 3 weeks. Ciliature of frontal gill surfaces consisted of distinct lateral, frontal and laterofrontal ciliary tracts. At juvenile excystment, the foot is ciliated ventrally and covered with microvilli dorsally. By 8 weeks, the byssus complex was formed. Acute toxicities (48-h) of cadmium to laboratory-reared larval and juvenile L. ventricosa were evaluated. Young juvenile stages were more sensitive than older juvenile or larval stages. Growth and histopathological effects of cadmium on juvenile L. ventricosa were evaluated using a 7-d static renewal test. Morphometric analyses of the shell revealed significant reductions in growth of anterior juvenile shell. Histopathological alterations appeared more severe for mantle, ganglia and digestive gland tissues as compared to foot and muscle tissue. Exposures to 30-100 [mu]g Cd/L resulted in dissolution of the crystalline style in several juveniles. Histological examinations indicated that juvenile feeding was reduced, lipid catabolism was altered and mucous hypersecretion may have occurred.