Nymphs and adults of the German cockroach, Blattella germanica, tend to aggregate. The effect of six different relative humidities (RH) on the aggregation behaviour of groups of 30 first instars was tested. The results show that grouping is denser under lower RHs than under higher humidities. The mean interindividual distances (nearest neighbour distances) proved to be inversely proportional to saturation deficiency (SD). The effect of aggregation behaviour in regulating water loss by evaporation is discussed. Under deprivation of food and drinking water survival times of the nymphs depend on RH. For five humidity levels tested the longevity (defined as MT(50)) proved to be inversely proportional to SD. The correlation is best fitted by a hyperbola.