The sensitivity of estuarine bacteria to organotin compounds (OTCs) was examined using the agar plate method by counting colony forming units (CFU) for estuarine water samples gathered at Aburatsubo and Kachidoki. Inhibition of bacterial colony formation occurred in a concentration range of 2 to 40μg Sn/l of bis-tributyltin oxide (TBTO) and triphenyltin chloride (TPT) regardless of media composition, inoculation method, or sample water. Similar numbers of TBTO- and TPT-resistant bacteria were counted by the MPN method at the inhibitory concentration range, indicating that the agar plate method was applicable to tests of the sensitivity of natural populations of estuarine bacteria to OTCs. The inhibitory activity of seven kinds of TBTO- and TPT-related compounds was compared. The results revealed that dialkyltins were most toxic, followed by trialkyltins, monoalkyltins, and tetraalkyltins. This order is unique for bacteria, and has not to date been reported for any other organisms.