Clones of bacteria were assayed by adding broth cultures (yielding 106-107 cells/ml) to beaker cultures of healthy Venus mercenaria larvae. Only 2 clones caused extensive mortality; both were isolated from a moribund larva in a laboratory culture. One is a species of Vibrio, the other is a Pseudomonas. Larvae exposed to virulent bacteria and to antibiotics simultaneously were uninjured. Larvae were grown bacteria-free by the use of antibiotics on newly-fertilized eggs followed by pipette isolation. Virulence of the clones was confirmed. Effects of bacterial metabolites were tested by glass filtering broth cultures or by killing bacteria with heat. High concentrations of metabolites (corresponding to ca. 109 bacteria/ml in the larval culture) depressed larval growth but caused no extensive mortality during the experimental time.