The ampicillin resistance of clinical isolates of Shigella sonnei is due to β-lactamase production. Two kinds of resistance are found: low level, nontransmissible; and high level, tranmissible. The nontransmissible type of resistance results from a chromosomal mutation which increases the production of a β-lactamase that hydrolyses cephalosporins relatively rapidly and gives cephalothin resistance. The transmissible type of resistance is due to an R factor mediating the synthesis of a different β-lactamase that does not significantly hydrolyze cephalosporins or confer cephalothin resistance. One clinical isolate is shown to possess simultaneously both these mechanisms of ampicillin resistance.