Diurnal variations in the vertical distribution and swarming behaviour of Euphausia crystallorophias at Deception Island (61 degrees South, 66 degrees West) are described. Swarms were present at all times of the day and night. By day swarms were compact and deep, they were dispersed and neared to the surface by night. Significant net avoidance was noted by day. A twenty-fold discrepancy was found between abundance estimated from net hauls and acoustics. This difference is almost certainly because currently used Target Strength (TS) to size relationships overestimate TS and consequently undersetimate euphausiid biomass.