Excreta potentially provide parasites or predators with information about the presence of hosts or prey; hence, vulnerable individuals experience strong selection to minimize danger from this source. Alternatively or additionally, excreta could alert potential prey to a spider's presence. Adult female crab spiders Misumena vatia (Clerck 1757) exhibited a strong reluctance to excrete when retained under tightly confined conditions. Only 5% of regularly fed individuals (1% of total observations) excreted over observation periods of as many as 50 days while confined in 7-dram vials (5 cm high, 3 cm diameter). Individuals retained large amounts of excreta during this time. However, when released upon vegetation over two-thirds of them excreted within 5 min, after moving to the distal end of a leaf or petal such that the excreta fell below them onto lower vegetation or the substrate. In the field they showed little tendency to excrete close to their hunting sites. The ability to retain excreta should serve this relatively sedentary species well in situations where it suffers high rates of attack or may reveal its presence to potential prey.