SUMMARY A mentally retarded child with congenital malformations is described in whom a high proportion of cells contained a small ring chromosome derived from a no. 18 (Denver classification). The variability in the morphological appearance of the ring chromosome is illustrated and its stability is discussed in relation to other instances of ring chromosomes in both man and other organisms. The writers wish to thank Dr Margaret D. Baber and Dr Ursula James for their co‐operation during this investigation, Mr A. J. Lee for preparing the figures and Miss Penelope M. Carr, Miss Linda Z. Gorman and Miss Barbara J. Warland for technical assistance. Research grants from the British Empire Cancer Campaign (N.H.K.), from St George's Hospital (M.K.L.) and from the United States Public Health Service (RG‐6984) (J.R.E. and R.M.) made this study possible.

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