The mollusc fauna of a medium-sized lowland river Krzna, which is almost entirely regulated and affected by urbanisation, was surveyed. The aim of the study was to determine if negative effects of urbanisation were evident in mollusc communities: their species richness, diversity and abundance, as well as composition and dominance patterns. Thirty one living mollusc species and empty shells of another two species were recorded. The simplification of the dominance pattern observed especially within and below Łuków and within Międzyrzec Podlaski may point to a negative effect of urbanisation. A decrease in species richness and diversity in response to urbanisation was recorded within Międzyrzec Podlaski, as well as below Łuków and Biała Podlaska. The mean abundance showed an increase in two urban and two downstream stretches. The relatively ambiguous response of malacocoenoses to urbanisation may result from the elimination of sensitive mollusc species by stressors associated with regulation of the river and release of untreated or insufficiently treated sewage in the past decades. Both the regulation of the river and urbanisation may have contributed to the negative effects on the malacocoenoses, but separation of the influence of these two anthropogenic stressors may be very difficult.