Organotins are widely used in agriculture and industry. They are toxic to a variety of organisms including bacteria, although little is known of their physiology and ecology. Bacteria resistant to six organotins-tributyltin (TBT), dibutyltin (DBT), monobutyltin (MBT), triphenyltin (TPT), diphenyltin (DPT), and monophenyltin (MPT)-were isolated from Boston Harbor sediments, Massachusetts, USA. Bacteria resistant to each of the organotins, except DPT, were isolated directly from estuarine sediments. Viability of the organotin-resistant bacteria on serial transfer in the laboratory ranged from 80 to 91%. Each isolate was screened for resistance to the other organotins. All of 250 isolates were resistant to at least two organotins. No DPT-resistant isolates were found on initial isolation on DPT, although there was DPT resistance among the other organotin-resistant bacteria. Eighty percent of TBT-resistant bacteria were TPT-resistant, suggesting that antifouling paints containing TPT will not be a suitable substitute for TBT in paints designed to inhibit microbial biofilms. Debutylation reduced toxicity in some cases while dephenylation did not. Thus, even though trisubstituted organotins are generally believed to be more toxic than di- or monosubstituted organotins, this may not always be the case, and more than one mechanism of resistance may be involved. All the bacteria were resistant to at least six of eight heavy metals tested, suggesting that resistance to heavy metals may be associated with resistance to organotins.