The concentration and chemical speciation of arsenic in the waters and sediments of Lake Ohakuri, New Zealand were examined. Mixed microbial populations from the sediments were tested in vitro for their ability to mediate redox transformations of inorganic arsenic. Under aerobic conditions the mixed microbial cultures were found to be able to reduce arsenic(V) to arsenic(III) and also to oxidize arsenic(III) to arsenic(V). Under anaerobic conditions only reduction of arsenic(V) to arsenic(III) was observed. Four species of sediment fungi were isolated, grown aerobically and all were found capable of reducing arsenic(V) to arsenic(III). The role of microbial heterotrophs in determining the observed mobility and speciation of arsenic in Lake Ohakuri is discussed. © 1986.