Up to 80% of faecal Escherichia coli strains are able to produce type 1 pili. These filamentous bacterial surface organelles, which mediate mannose-sensitive attachment to mammalian epithelial cells, are also conserved throughout the Enterobacteriaceae. As a potential explanation for their prevalence among intestinal isolates of enteric bacteria, it has been widely speculated that type 1 pili are important for adherence to the host's intestinal mucosa. However, conclusive evidence for this idea is lacking, and there are reasonable grounds for doubting such an effect. Permanent interruption of type 1 piliation in previously pil+ E. coli (by directed mutagenesis of pilA, the gene coding for the major structural subunit of type 1 pili) does not diminish the density of intestinal colonization in individual animals. Rather, as we demonstrate here, this lesion results in a dramatic decrease in transmission of E. coli K1 from experimentally colonized neonatal rats to their littermates. The enhanced communicability associated with type 1 piliation suggests a heretofore unrecognized explanation for the prevalence of type 1 pili among intestinal E. coli; one that does not necessarily require the direct action of these organelles at the intestinal mucosa.