The Malaysian state of Sabah, in northern Borneo, started massive monoculture forest plantations of fast-growing introduced tree species in the mid-1970's to replace part of the harvested tropical rain forest. Many people, particularly conservationists in the West, are very much against this sort of reforestation as they fear it would spell a permanent loss to the Bornean rain forest biodiversity. This project was carried out at the more established forest plantations of Sabah Softwoods Sdn. Bhd. in Brumas from 1991 to 1993, where fast-growing exotics namely Acacia mangium, Eucalyptus deglupta, Gmelina arborea, Paraserianthes (=Albizia) falcataria, Pinus caribaea, were studied to assess their biodiversity and these plantations were compared with the natural regenerating logged-over secondary forest in Brumas, as well as the primary forest in Danum Valley, by using light-trapped macromoths as indicators. The method of light-trapping as a reliable means of capturing moths was supported by canopy knockdown in the form of mist-blowing. Results obtained showed that for the year-long (January-December 1991) light-trap samples, the biodiversity values, as represented by Williams alpha (higher the value, higher the diversity), were unexpectedly high in the various plantation forests. Their alpha values ranged from the lowest in Acacia mangium with 208.14+-9.22, to the highest in Eucalyptus deglupta with 330.85+-16.37 which was even higher than the natural secondary forest with 314.53+-11.99, and certainly not inferior to the published values (300 to 350) from undisturbed Bornean forest of similar altitudes (below 500m). For the shorter month-long subsidiary samples (October/November 1992, January/February 1993), the alpha values of the samples from the lowland primary forest in Danum were not necessarily higher when compared with the similarly sampled disturbed forest habitats in Brumas, but despite its small samples, Danum produced some 33 species of macromoths which were never collected out of the 1680 species ...