An assay was developed which identifies individual bacterial tumbles and so allows rapid, quantitative measurements of tumble frequency in free- swimming bacteria. Tumble frequency is modulated by cells to enable chemotaxis. Mutations in the chemotaxis signal transduction pathway typically have phenotypes of altered tumble frequency. The purpose of this assay is to quantitatively measure steady-state tumble frequency to enable comparisons of mutant strain phenotypes. It was developed using Escherichia coli but should be applicable to other species with a peritrichous flagellation pattern, such as Salmonella typhimurium. Tumbles are defined by a combination of the parameters late of change of direction and swimming speed, with a rapid change of direction defining the beginning of a tumble and increased swimming speed defining the end. These parameters have previously been shown to be correlated with tumbles in general but not used to identify discrete tumble events. The computer assay was validated by comparing its results with manual observations by eye. The assay was intended to be most sensitive to swimming patterns similar to wild type so as to resolve subtle changes which would result from partial-function mutations. It quantitatively detects extreme behavioral phenotypes as well and can be modified to increase resolution at either extreme if necessary.