Coliform bacteria from raw and treated sewage in a mechanical-biological treatment plant were examined for resistance to four to six antibiotics, and randomly selected isolates were identified. The results indicate that certain coliform species are eliminated at lower rates than others by the treatment. Klebsiella sp. became more frequent, whereas the proportion of Escherichia coli diminished. A comparison of raw and treated sewage clearly reveals that the resistance patterns are transient characteristics, and resistance percentages may either decrease or increase following passage through the treatment plant. Mating of multiresistant isolates with E. coli K12 donor confirmed that some of the resistance traits are borne on conjugative plasmids. Multiple antibiotic resistance (MAR) indexing revealed that resistance levels of E. coli decreased during waste water treatment, but increased in total coliforms.