We have investigated the pulsation behavior of InGaAsP semiconductor lasers with a proton‐bombarded segment. These lasers emit picosecond (30–70 ps) pulses at gigahertz (0.6–3.0 GHz) rates. An antireflection‐coated diode in an external cavity is passively mode locked and multistable; as many as four co‐existing states are observed. Interlocking hysteresis loops are observed in the pulsation frequency, pulse width, and output power as functions of the bias current. A delayed feedback model explains qualitative features of the multistable mode locking. To our knowledge this is the first report of multistability of laser pulsation.