ABSTRACT: Multi-frequency acoustic data (18, 38 and 120 kHz) were collected in conjunction with pelagic trawl sampling for gelatinous macrozooplankton during a cruise to the Namibian Benguela in September 1999. Sampling focused specifically on thescyphozoan Chrysaora hysoscella and the hydrozoan Aequorea aequorea, both of which occur in large numbers, are probably of major ecological importance, and physically hamper pelagic fishing and diamond extraction activities. C. hysoscellawas detected predominantly at an inshore station and A. aequorea was found in greatest abundance further offshore in deeper water. Echo-sounder observations were linked directly to net catches, and relationships between catch density (numberof individuals m^-3) and nautical area scattering coefficients (s_A) at each frequency were determined for both species in order to estimate target strength (TS) using the comparison method. TS for C. hysoscella (meanumbrella diameter 26.8 cm) was -51.5 dB at 18 kHz, -46.6 dB at 38 kHz and -50.1 dB at 120 kHz; for A. aequorea (mean central umbrella diameter 7.4 cm) TS was -68.1 dB at 18 kHz, -66.3 dB at 38 kHz and -68.5 dB at 120 kHz. These TS values comparedfavourably with previously published estimates for related species. Jellyfish were caught at high numerical densities (maxima 3 C. hysoscella per 100 m³, 168 A. aequorea per 100 m³). These high densities, combined withthe not unsubstantial TS at frequencies used for fisheries surveys, imply that jellyfish could potentially bias acoustic estimates of fish abundance. We suggest a simple multifrequency approach that could be used to discriminate between echoes fromjellyfish and some commercially important pelagic fish in the northern Benguela ecosystem.