Fish micronuclei tests were used to evaluate the potential of municipal wastewater sewage to cause genetic change in Bathygobius soporator. The levels of toxicity of municipal wastewater sewage were evaluated in populations of B. soporator originating from metal-contaminated and metal-free areas. Blood samples were taken from ten fish caught in sites with low anthropogenic influence to measure the levels of spontaneous micronucleated erythrocytes. An additional 30 fish specimens were captured to conduct laboratory testing. After a 30-day acclimation period, blood samples were collected under controlled conditions, while fish that were not sampled from blood were exposed to concentration of 10 % of the urban effluents for 10 days. Acute toxicity experiments were also carried out at different concentrations (0, 1, 3, 7, 10, and 30%) of urban effluent for 48-h in using fish specimens of the two sampled populations of B. soporator. The results of this study showed that mortality was threefold higher for fish exposed to the urban effluents in the population of metal-free area compared with fish from the metal-contaminated area. There were micronucleus inductions in fish after 10 days of exposures to the urban effluents and significant differences (p< 0.01) after only 10 days when comparing to spontaneous micronucleus levels. We concluded that were microncleus inductions from urban effluents leading to higher mortality of individuals from the population of metal-free area, and this was higher than for individuals of the population exposed to metal contaminants.