In addition to a soluble response, many invertebrates control bacterial infections by means of phagocytosis or melanotic encapsulation. In some insects, Escherichia coli growth is reported to be inhibited by aggregation/encapsulation. Soluble and phagocytic responses to bacterial challenge have been reported in ticks, but evidence of an aggregation / encapsulation response was reported only for inanimate (araldite) implants. Ticks were challenged by direct inoculation of bacteria into the hemocoel cavity. By plating, no viable E. coli were detected 6 h postinoculation. A direct fluorescence assay (DFA) revealed aggregated bacteria 1 h postinoculation. Furthermore, DFA showed aggregated bacteria at 6,24, and 48 h postinoculation associated with masses of tissue, presumably of cellular origin, suggesting events similar to those described as nodulation. These findings suggest that encapsulation/ nodulation may be an important component of the immune response in ticks.