In England and Wales, it has been found that bivalve species growing at the same site may show differences in the levels to which they accumulate contamination to the extent that they may fall into different classification categories. This study will initially examine the data at sites with two or more species and statistically (primarily using paired t-tests) assess the significance of any differences. It will then endeavour to characterize such differences and establish whether there are consistent trends across England and Wales. Where inconsistencies in possible trends may exist then the reasons for these will be investigated. The aim of this study is to determine whether it might be possible to rationalise monitoring programmes (e.g. monitor one species to represent two or more) where clear relationships in terms of E. coli concentration characteristics exist. This would have the benefit of conserving monitoring resources and perhaps lessen delays in classification by allowing species to be classified without specific monitoring. The main finding of this study was that the average E. coli accumulation in Mytilus spp. was between 1.4 and 3.4 times greater than in C. gigas.