Water is essential to life, yet many people worldwide do not have access to clean drinking water and suffer or die from preventable diseases caused by unsafe water. Microbiological water analysis is based on coliform bacteria, but these microorganisms can have their growth affected by others, such as Pseudomonas aeruginosa. A total of 60 isolates of P. aeruginosa was investigated in this study to obtain better knowledge about the relationship between P. aeruginosa pigments and the effect of antagonism against coliforms. Of these, 40 isolates were isolated from environmental samples (drinking water and soil) and 20 from clinical patients. Three environmental coliform isolates from water samples, two Escherichia coli and one Klebsiella pneumoniae, and an Enterobacter aerogenes were used in antagonism tests. The results have demonstrated that these bacteria were inhibited by P. aeruginosa by an effect known as antagonism. The inhibitory action of P. aeruginosa against coliforms was more effective when P. aeruginosa produced pyocyanin and/or pyoverdin.