The upper thermal tolerances of 12 New Zealand freshwater invertebrate taxa were compared using a laboratory lethality testing protocol. Temperatures that were lethal to 50% of the test organisms (LT50S), following acclimation to 15°C, generally declined over the 4 day period of the tests. LT50values after 48 and 96 h exposure ranged from 24.5 to > 34°C and 22.6 to 32.6°C, respectively, indicating that temperatures that occur in summer in many NZ streams and rivers may limit the distribution and abundance of some of these invertebrate species. Larval insects included both the most sensitive species (a plecopteran and two Ephemeroptera) and the most tolerant (the larvae of the elmid beetleHydorasp.) in this study. The two molluscs studied (Potamopyrgus antipodarum(Prosobranchia: Hydrobiidae) andSphaerium novaezelandiae(Bivalvia: Sphaeriidae)) showed high thermal tolerance, whilst two crustaceans (Paratya curvirostris(Atyidae) andParacalliope fluviatilis(Eusiridae)) were moderately tolerant. The effect of a higher acclimation temperature (20°C) on thermal tolerance was also investigated for the three most sensitive species. This appeared to increase the thermal tolerance of the two Ephemeroptera{Deleatidiumspp. andZephlebia dentata(both Leptophlebiidae)) but to reduce the tolerance of the plecopteranZelandobius furcillatus(Gripopterygidae).