Extensive research has shown that microorganisms exhibit increased resistance due to clumping, aggregation, particle association or modification of antecedent growth conditions. During the course of investigating a major waterborne V. cholerae outbreak in Peru, U.S. EPA investigators discovered an extremely rough form of V. cholerae that was highly resistant to disinfection by chlorine. This form of V. cholerae also tends to form large aggregates, which contribute to the resistance. Particle counting was used to characterize each of the cultures examined. Statistical analysis revealed that the rugose, or rough, strain of V. cholerae was much more resistant to disinfection than the common, or smooth, strain. Resistance was attributed to both aggregation and a mucoid coating. The larger aggregates associated with the rugose organism are of a size as to be easily removed by filtration, but if the variant were to contaminate a distribution system through a line break or cross connection it would be difficult to control by chlorine disinfection.