In situ bioremediation is now being considered as an operational oil-spill countermeasure technology. While the effects of treatment strategies on microbial populations have been studied extensively, information on bioconcentration and effects on survival, growth and reproduction of higher level macrobiota are limited. Mystery snails, Viviparus georgianus, are attractive wetland biomonitors because they are abundant, short-lived, dioecious, ovoviviparous, easy to collect and grow rapidly during summer months feeding on sediment debris. V. georgianus was used as biomonitors in a controlled oil spill experiment at a wetland site along the St. Lawrence River (Ste. Croix, QC) to assess the impact of crude oil and efficacy of bioremediation treatments. Snails were placed at various time intervals in special enclosures deployed within five treatments and control background plots (n=50/treatment/collection time). Treatments consisted of A: oiled control (natural attenuation), B: as A + ammonium nitrate + triple superphosphate + culling of plants, C: as B but plants left intact, D: as C but sodium nitrate instead of ammonium nitrate, and E: as C with no oil treatment. Although snails could survive in the presence of oil for up to two months, fertilizer treatments brought about increased mortality. Generalized tissue damage with edema and hemocytic infiltration was seen consistently in snails from treatment D and reproduction was impaired in all treatments with or without oil. These findings disclose the need to further evaluate bioremediants in oil-spill response operations for appropriate recovery.