Digestion with Sal I facilitated the subclassification of 41 strains of Campylobacter jejuni into seven types, and digestion with Sma I enabled subclassification into twelve types. Sma I was potentially more useful for the detection of variability among the 41 strains, but both restriction enzymes seemed to be potentially useful for detecting variability among crossed-field gel electrophoresis profiles of the strains. The results clearly demonstrated that C. jejuni strains from different sources and with different routes of transmission had invaded the three farms investigated. At farms Sa and Ai, approximately 70% (23 strains) of isolates of C. jejuni (33 strains) were subclassified into the two major genotypes (I and II) on the basis of cleavage profiles with both Sal I and Sma I. These two major genotypes appeared to have invaded, expanded in and occupied the two chicken farms. All nine strains from farm Ai belonged genotype I.