The resistance types of Citrobacter freundii were investigated. The strains were divided into three groups according to susceptibility to penicillin derivatives. Group 1, comprising the ampicillin-carbenicillin sensitive strains (A-s/Ca-s), constituted 22%. Group 2 (ampicillin-resistant (A-r)/Ca-s) constituted 45% and group 3 (A-r/Ca-r) 32%. Two strains (1%) were recorded as A-s/Ca-r. Group 3 contained a higher proportion of multi-resistant strains than groups 1 and 2. The log2IC50 with ampicillin, carbenicillin, cephalothin and mecillinam in the tube-dilution method were determined for 32 strains. Groups 1 and 2 differed only as regards cephalothin resistance. The strains of group 3 were more A-r than the group 2 ones. The mecillinam susceptibility did not differ between the three groups. No strains in group 2 could transfer A-resistance. In group 3 ten out of 16 strains were able to transfer A-resistance. Resistance to sulphonamide, tetracycline, and streptomycin was also transferred, but with low frequency. The A-r/Ca-r strains could be divided into two subgroups, probably reflecting different modes of developing β-lactam resistance. One group was very A- and Ca-r, less cephalothin-r, more mecillinam-r and could transfer A-resistance. The other group had very high cephalothin-resistance and could not transfer A-resistance. Citrobacter freundii and Enterobacter cloacae are very similar as regards resistance conditions.