Koalas Phascolarctos cinereus on St Bees Island displayed two significantly different patterns of tree species utilisation. Utilisation by day was complex with 36.5% in Eucalyptus tereticornis, Forest red gum, and 63.5% in a suite of non-eucalypt species. By night, utilisation was 80% E. tereticornis. Analysis of cuticle fragments in faecal pellets revealed almost 100% of the diet was composed of E. tereticornis. Nocturnal tree species utilisation differed from day use and from species occurrence in the diet. Direct observations of koala over 24 hrs found that 78.5% of feeding occurred at night. It was concluded that (1) observations of tree species use by day provided no indication of the composition of the koala diet on St Bees Island, (2) although observations of tree use by night provided a better estimate of the diet there were significant differences - probably associated with the nocturnal life of the koalas, and (3) it seems likely that analysis of cuticle fragments within faecal pellets is the most effective technique for understanding koala diet. The reliance of the St Bees Island koala population on a single eucalypt species was demonstrated.