Cotesia rubecula (Marshall) was first released in New Zealand for control of Pieris rapae (L.) in December 1993. It has since been released in nine regions and is established in eight of these, from Northland to Southland (35° 15′ S to 46° 10′ S). Natural geographic spread of C. rubecula has averaged ≈2 km/yr in the Pukekohe area in South Auckland. Death rates due to parasitism ranged from 48 to 97% of host larvae at selected study sites, but were lower at most commercial sites. Three paired comparisons of sites with and without C. rubecula showed that the parasitoid reduced the survival of P. rapae larvae and limited the density of fifth instars. Although C. rubecula reduced parasitism by Cotesia glomerata (L.) in sites where they were both present, total parasitism of P. rapae was increased at these sites. Some hyperparasitism of C. rubecula by Tetrastichus galactopus was observed, but it does not appear to be limiting establishment of C. rubecula.