Summary Following a definition of bacterial L‐forms and a literature review on introduction and reversion of bacteria to and from L‐forms, respectively, the results of a study comprising 587 samples (316 pigeons and 271 pigeon eggs) are reported. As a control 25 free living pigeons and 25 eggs of those pigeons were used, because antibiotic treatment of these could be excluded. From 79 samples (75 pigeons and 4 eggs) Salmonella typhimurium var. copenhagen (STMC) was isolated (non from the urban pigeons), of them 11 in the L‐form from joints and organs and 3 from eggs. 325 of other bacterial isolates were found as L‐forms (=approx. 40%). Out of 168 serum samples investigated, 33.9% showed antibodies against STMC. Corresponding antibodies could only be demonstrated in 73.4 % of the pigeons with a STMC isolate. The occurrence of L‐forms explains resistance to therapy and the failure of vaccines. The high frequency of L‐forms is probably an indication of an inconsequential use of antibiotics in managing pigeon diseases.