Australorbis glabratus and other aquatic snails exposed to either of two tetracyclines in water absorbed these compounds and incorporated them, as fluorophores, into the shell. Shell fluorescence was detectable in a significant proportion of snails for 5 to 8 weeks following exposure to tetracycline, and individual shells exhibited fluorescence for 12 weeks. Although exposure to tetracyclines resulted in transient inhibition of growth and fecundity in A. glabratus, rapid recovery of both functions occurred after drug withdrawal. The capacity of A. glabratus to become infected with Schistosoma mansoni was not influenced by prior exposure to tetracycline HC1. A limited field trial, concerned with recapture of tetracycline-marked snails, is described. Snails exposed to certain dyes (acridine orange, uranine, rhodamine B) or to a pentahydroxyflavone (quercetin) did not develop shell fluorescence. Moreover, the toxicity of the dye compounds for snails was marked. Recent studies demonstrating that tetracycline compounds localized in bone and other tissues in which calcium deposition occurs (Milch et al., 1957, 1958; Bevelander and Goss, 1962) suggested that these compounds might also become incorporated into the growing shell of snails. The capacity of localized tetracycline complexes to exhibit long-term fluorescence further suggested their possible use to mark snails. Fluorescent marking techniques have long been used by entomologists (Bailey et al., 1962), but have not been applied to molluscs. This study was undertaken to determine if tetracyclines, or certain other substances, would be taken up by Australorbis glabratus and by other aquatic snails and thus serve as fluorescent markers. MATERIALS AND METHODS In preliminary work, several compounds known to fluoresce under ultraviolet light (UV) were first evaluated to determine those that might be absorbed from water and incorporated into snails as fluorophores. These compounds included Received for publication 13 July 1964. * These studies were supported, in part, by Grants AI-00513 and 5 Tl AI 46 from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, U. S. Public Health Service, Bethesda, Maryland. t Contribution from the University of Michigan Biological Station. tetracycline HC1 (Achromycin?, Lederle),1 oxytetracycline (Terramycin?, Chas. Pfizer and Co.), and the dyes acridine orange, water soluble uranine, and rhodamine B (all from National Analine Co.). Quercetin (Nutritional Biochem. Corp.), a pentahydroxyflavone, was also tested in view of the report (Ribelin et al., 1960) that its inclusion in the diet of rats resulted in fluorophores in the bones. Groups of ten snails (9 to 12 mm shell diameter), either of a Boston strain of Helisoma anceps anceps or a Puerto Rican strain of A. glabratus, were placed in separate beakers, each containing 1 liter of charcoal-filtered water. Beakers, except for the 1st day, were aerated and supplied with romaine lettuce. Some groups of snails were exposed to acridine orange, uranine, or rhodamine B for 5 consecutive days at original dilutions of 1, 10, or 100 ,tg/ml. Quercetin (1%) was mixed in a blender with the ingredients of Standen's alginate food mixture (Moore et al., 1953), and the mixture was gelled by injection into 2% CaC12. Each group of snails was fed approximately 10 g/week of this food for 3 consecutive weeks. In other preliminary studies, 10, 15, or 20 mg of the two tetracyclines were added to beakers, containing 1 liter of water, daily for 5 days. These experiments led to three others in which 10 mg of tetracycline HC1 (T-HC1) were added to a liter of water daily for 5 consecutive days. The amount of drug employed each day was divided into two equal doses and administered in the morning and afternoon. 1Tetracycline hydrochloride compounded for intravenous use was employed in all experiments. The preparations intended for intramuscular use contained procaine and proved more toxic.