IMMUNE defence against pathogens entering the gut is accomplished by lymphocytes in the gut-associated lymphoid tissue (GALT), a major compartment of the immune system¹. The GALT, comprising Peyer's patches, lamina propria lymphocytes and intra-epithelial lymphocytes of the intestine, is populated by lymphocytes that migrate there from the vasculature^2,3. Here we report that, in mice deficient for the β7 integrin subfamily of adhesion molecules, the formation of the GALT is severely impaired. This is probably due to a failure of β7^–/– lymphocytes to arrest and adhere to the vasculature at the site of transmigration into the GALT.