In preliminary trials of an X-band side-looking airborne radar (SLAR), surface schools of fish were seen as distinct targets. Radar backscatter from the fish schools was influenced by the behaviour of the fish and the angle of the radar beam relative to the prevailing wind and wave conditions. Detection of targets was partly masked by radar backscattering from modulation of the Bragg-scale waves by oceanic phenomena, radiometric variations due to aircraft motion, and ‘bursts’ from breaking waves. Fish spotting was carried out by three methods: (i) visually, (ii) with the grey-scale image on a monitor supplied with the radar, and (iii) with a real-time display produced by a microcomputer interfaced lo the SLAR processor unit. Target signals were enhanced on the microcomputer in real-time using a difference filter, and colour-coding was used to differentiate the strength of targets. This procedure revealed several weak backscattering targets not visible in the unenhanced black-and-white display. The 'boiling’ surface disturbances created by frenzied feeding schools of jack mackerel were detected visually within a few kilometres of the aircraft and by the radar out to a range of 30km. Non-feeding schools of southern bluefin tuna created a surface rippling, similar to that caused by light winds, which could not be detected without image enhancement.