In the PNS, myelin basic protein (MBP) appears not to be essential for myelination, for in shiverer (shi) and mld mutant mice peripheral nerves, where MBP is not or only poorly expressed, myelination occurs normally. Only a few morphological abnormalities, i.e. reduction in axon calibre and myelin sheath thickness, and aberrant Schwann cell-axon contacts, have been reported. Here, we document a consistent difference between shi and wild type (wt) myelinated sciatic nerve fibres. The number of Schmidt-Lanterman incisures seen in longitudinally and transversely-sectioned sciatic nerves, or in teased fibres stained for the presence of F-actin, is dramatically increased in homozygous shi mice. With both methods, a twofold increase in Schmidt-Lanterman incisure number is seen in 15-day-old mice, the earliest time examined. The increase is slightly greater in nerve fibres from 30- and 90-day-old mice. The overproduction of Schmidt-Lanterman incisures in shi occurs in spite of the fact that the mean diameter of myelinated fibres in shi sciatic nerves is smaller than in wt sciatic nerves. These results lead us to suggest that the increase in Schmidt-Lanterman incisure density in shi compensates for a defect in Schwann cell-axon communication.