The boring sponge Cliona celata poses an ongoing threat to the quality of eastern oysters Crassostrea virginica cultured in Baie St-Simon, Shippagan, New Brunswick, Canada. Although grown primarily off-bottom, 25–30% of cultured oysters develop severe sponge infections, leading to the culling of otherwise marketable individuals. The main objectives of the current study were to identify the areas with the highest prevalence of infected oysters, determine the primary mode of sponge infection, document the rate of shell bioerosion, and develop an effective treatment for eliminating the sponge without harming the oysters. A bottom survey of wild oysters throughout the bay indicated a high prevalence of sponge infection in certain areas, possibly related to seasonal salinity patterns. No evidence of sponge infection via shell-to-shell contact, or proximity to wild oysters, was detected during the winter months when oyster shells were deployed in bags lying on the bottom. Histological sectioning of sponge tissues in severely infected wild oysters revealed the presence of developing gametes in early June, mature eggs and sperm from mid June to mid July, and empty follicles in late July. Larvae were never successfully identified in plankton tows, but the sequential deployment and retrieval of experimental shells corroborated that June/July was the peak infection period. By the end of October, single infections measured approximately 0.05–0.10 cm2, with evidence of shell bioerosion and papillae development. Monitoring of sponge growth in 70–90-mm shells (upper valve) indicated that a 5-cm2 infection will grow at a rate of approximately 15 cm2/y, rapidly occupying the 21-cm2 valve of a marketable oyster (64-mm shell length). Comparison of various treatment strategies indicated that a 6-min brine dip (>90% NaCl saturation) was completely effective at eliminating the boring sponge without harming the oysters. A program of brine dipping every second year appears to be sufficient to control the development of this pest organism in cultured oysters.