A new species of Hirondellea was trapped in large numbers (N=441) in the Atacama Trench (Pacific Ocean) at 7800 m depth. The species accounted for 47% of the total number of amphipods collected and was the second most abundant amphipod after Eurythenes gryllus. Adult males and females reached maximum lengths of 13·5 and 14·1 mm. Length–frequency histograms showed no clear evidence of growth classes, but instars could be determined by using a combination of morphological criteria and size–frequency data. Growth factors based on mean lengths of successive instars ranged from 1·09 to 1·24 except at moults immediately prior to maturity when factors were in the range 1·02–1·04. Females moult from the pre-adult to the adult instar, without body growth. Moulting modalities of immature males are unclear. Equations obtained from regression analysis between wet weight and total body length indicate an allometric growth. Females grow at a rate of 10% higher than males, and both females and males display growth rates lower than those observed for juveniles, but growth increments at each instar of females are not significantly higher than those of males. The relationship between total body length (L) and wet weight (W) of all instars of Hirondellea sp. nov. is expressed by the power curve W=0·012 L3·279 (W in mg and L in mm).