This paper describes a study of behavioural response in terrestrial isopod Oniscus asellus when offered two food pellets of different quality. One group had a choice of sterilised food and food pellets covered with mould. The other group had a choice of uncontaminated and cadmium-dosed food. During the behavioural test, the animals were monitored by a video camera and each visit to food pellets and time spent around it was counted. The results show that animals spent significantly less time near sterilised and cadmium-dosed food than with uncontaminated or mould-covered food. Discrimination between offered food pellets showed that avoidance behaviour can be used as an indicator of different food quality.