SCUBA records on the distribution and abundance of macrophytes in Lake Rotorua show marked changes between 1980 and 1988. Extensive weed beds ofLagorosiphon major(Ridley) Wager, present since the mid‐1950s, disappeared from the lake by 1982. The loss of theL. majorweed beds coincided with marked reductions in water clarity in the early 1980s and with a storm in April 1982.Egeria densaPlanchon spread rapidly from a small infestation in July 1983 to become the most abundant macrophyte in the lake by December 1988 (occupying about 475 ha), equivalent to the historicL. majorweed beds. We predict thatE. densawill be dispersed to become a major problem weed in other Rotorua lakes in the future.