The motility of four strains of Campylobacter jejuni in solutions of varying viscosity was measured and compared to that of a number of conventional rod-shaped bacteria (CRSB). All the bacteria tested showed an initial increase in velocity in the low viscosity solutions--between 1 and 3 centipoise (1 P = 0.1 Pa s). However, only the campylobacters were actively motile in highly viscous solutions with velocities ranging from 60 to 100 micron s-1. All strains of C. jejuni tested showed three separate peaks of motility as the viscosity of the solution was increased. A higher proportion of C. jejuni cells exhibited longer path lengths when the viscosity of the surrounding medium was increased from 1.4 to 57 cP. The findings of the study suggest that C. jejuni has a motility suited to movement in a viscous environment, and that this ability might provide the organism with an ecological advantage when in intestinal mucus. It is proposed that the mechanism of motility changes depending on the viscosity of the supporting environment.