Monitoring of heterotrophic nitrogen-fixing bacteria and presumptive coliforms in two Libyan sewage treatment plants showed that tertiary treatment of effluents by chlorination killed these bacteria. However, the irrigation reservoirs which received the treated effluent contained large numbers of both types of bacteria. As the proportion of nitrogen-fixers to presumptive coliforms was much less in the sewage than in the irrigation reservoirs it is probable that the irrigation water became contaminated by a source other than sewage.