Differences in the target strength of adult (36 to 60 mm) Antarctic krill (Euphausia superba) at 38 and 120 kHz have been inferred from differences in the mean back-scattering strength of swarms simultaneously insonified at these two frequencies in studies off the South Orkneys and Elephant Island in March 1990. Back-scattering strengths at 120 kHz ran consistently about 7 dB higher than at 38 kHz, a difference which was regarded as too large to be explained by possible experimental error, and which was therefore attributed to real differences in average target strength at these two frequencies. The results are in good agreement with recent experimental work on the target strength of encaged E. superba at 38 and 120 kHz but are in major conflict with the 120 kHz - to - 50 kHz target strength conversion factor used at the Post-FIBEX Acoustic Workshop in 1984.