Ecological studies were made on macrophyte-associated invertebrates at 13 sites within the macrophyte zone of Lake Grasmere, Cass, Canterbury, South Island, New Zealand (43°05'S, 171º45’ E; 583 m a.s.l.). A pilot survey (14 April 1976) and a quantitative sampling program (September 1976 - October 1977) were carried out using a new cylinder-sampler (which is described). Non-quantitative samples were taken also to provide additional information. About 113 species of aquatic invertebrates were collected from Lake Grasmere and its immediate environs including at least 75 species that were associated with submerged macrophytes. Chironomidae (17 spp.), Crustacea (16 spp.), and Trichoptera (12 spp.) were best represented. Thirty species were new records for the Cass district. Cluster analysis was used to simplify the analyses of data from the quantitative sampling program and dendrograms of site groups and species groups were derived. Four site groups were delimited on the basis of similarity in the composition of associated invertebrate communities. Ten species groups were identified on the basis of each species' distribution. The characteristics of the site groups and species groups are discussed. The 13 taxa of species group 6 occurred at all sites sampled and comprised over 95% (by numbers) of the fauna at each site group. Overall, Mollusca (mainly Potamopyrgus antipodarum) (55.9%), Crustacea (16.4%) and Coelenterata (Chlorohydra viridissima) (14.8%) were the numerically dominant groups on macrophytes. Seasonal changes in invertebrate communities at the site groups were considered using diversity indices and community similarity indices. The trophic relationships of Paroxyethira hendersoni, P. tillyardi, Hudsonema amabilis, Triplectides cephalotes (Trichoptera), Nymphula nitens (Lepidoptera), Xanthocnemis zealandica (Odonata) and P. antipodarum (Mollusca) were investigated using faecal analysis. Emphasis was placed on size- or instar-related dietary changes. N. nitens and T. cephalotes fed primarily on tissue of aquatic macrophytes; the two Paroxyethira spp. and P. antipodarum were herbivorous browsers on periphytonj X. zealandica was a predator (especially on Oligochaeta, Cladocera, Acarina and P. antipodarum); and H. Amabilis was omnivorous. Notes on the gut contents of Chironomidae are presented. Life-history information was collected for seven species of Trichoptera, N. nitens and x. zealandica. Most species had slow seasonal life-histories, with emergence during spring and summer. Exceptions were Oxyethira albiceps and P. hendersoni (non-seasonal, perhaps bivoltine) and x. zealandica (non-seasonal, predominantly a three year life-history). Taxonomic studies included the production of keys to larvae of Hydroptilidae and Chironomidae and adult males of the New Zealand Chironomidae. The fifth instar larva of Paroxyethira tillyardi is described and the habitats and distributions of the New Zealand species of Hydroptilidae are discussed. The male of Eukiefferiella sp. (Chironomidae: Orthocladiinae) is described; illustrated notes on chironomid larvae and pupae from Lake Grasmere are presented and problems in chironomid systematics are discussed.