The Yellow Sack Spider (Cheiracanthium punctorium) is a well-known species native to the Czech Republic. Several reports of its painful bite have gained much publicity. The species was considered to be very rare until the end of the 20th century. We provide information on the distribution, spread and ecology of this spider in the Czech Republic. Several cases of biting and its subsequent symptoms are described, and the toxicity of the spider is discussed. and Ondřej Košulič, Jan Korba, Jan Dolanský.