The koaro (Galaxias brevipinnis) is important in New Zealand's whitebait fishery but aspects of its ecology and life-history are poorly understood. Factors affecting koaro distribution and life-history were investigated in Manson Creek, a lake tributary stream. Six 20 metre sites were electrofished during the summers of 199912000 and 2000/0 1. All koaro > 90mm (FL) captured were tagged. Mean koaro size and density were higher at upstream sites. Brown trout (Salmo trutta) abundance was significantly lower upstream, but trout were larger upstream. Koaro distribution can be partly attributed to abiotic factors (more riparian forest and pools upstream). However, interactions with brown trout could also affect their distribution. All koaro were recaptured at the site of initial tagging indicating they utilised a well defined homerange. Mean growth of recaptured koaro was greatest at upstream sites, where koaro density was lowest. Juvenile koaro captured during autumn were the result of spring spawning. Koaro interactions with juvenile brown trout and habitat preferences (shade and velocity) were tested in stream channels. Koaro showed no microhabitat preference even in the presence of trout. However, koaro spent significantly more time alone in trout treatments compared to troutless treatments. Thus, trout affected koaro microhabitat choice more strongly than the microhabitat type. The influence of small and medium sized brown trout on the growth and mortality of small and large koaro was investigated during a one month growth experiment. Mean growth of small koaro was less in trout treatments and least in treatments containing the largest trout, suggesting that trout negatively affect growth of small koaro, possibly through competitive interactions. Less koaro survived in the largest trout treatments, suggesting that larger trout may have preyed upon smaller koaro. This study was undertaken to expand on the knowledge of koaro ecology and life-history in a landlocked population, and how the introduced brown trout may affect distribution, growth and survival of koaro.