Tubificidae are aquatic annelids which occur abundantly in organically polluted waters. The anterior end of their body is buried in the sediment, while the posterior end moves in the water. This posterior end is (partly) retracted in the sediment in the presence of defined concentrations of pollutants. The retraction can be followed visually (qualitative) or can be monitored by videocamera and quantified by digital image analysis. The latter presents the advantage of a continuous recording that is always processed the same way. Our preliminary results indicate that digital image analysis is an accurate technique for the quantification of the behavioural response of Tubificidae to pollutants. (C) 1999 Elsevier Science Ltd. till rights reserved.