Mussels (Hyridella menziesi) were tested for their suitability as monitors of bacterial contamination of natural waters because of their ability to accumulate bacteria while filtering the environment for food. Several questions were addressed : 1. Can E. coli be recovered from mussels immersed in low-density suspensions of bacteria. 2. What is the optimum time required to immerse mussels in suspension of bacteria. 3. How long will the mussels retain bacteria once uptake had occurred. 4. Is it possible to estimate the numbers of bacteria present in contaminated waters, Using the numbers of bacteria recovered from mussels. The Results of this study were : 1. It was possible to detect E. coli and Salmonella in mussels that had been immersed in water contaminated with these bacteria at densities as low as around 50 per 100 ml. 2. An experiment conducted to examine the ability of mussels to retain bacteria once uptake had occurred, showed there was rapid loss of E.coli (42.9%) over the first 4 hours and by 6 hours numbers had declined to very low levels (2.1 %). 3. Although there was some evidence of a correlation between the concentration of E. coli present in surrounding waters and the numbers recovered from mussels it was not possible to predict the numbers of bacteria in the surrounding water from analysis of the mussels. 4. Similar experiments were done using diluted sewage instead of pure cultures of E. coli and Salmonella. Results were similar to those obtained using pure cultures. E. coli could be recovered from mussels when present in the diluted sewage at concentrations as low as 8 / ml.