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The object of the paper is to give an upper bound for the length of the gaps that can occur in the essential spectrum of any selfadjoint operator which is generated by a generalised Dirac system of differential expressions in the Hubert space L'-(a, b). An estimate is also obtained for the limit point of the spectrum which has least absolute value. 1. Let r denote the matrix differential expression of order 2/? given by rf(t) = B<p'{t) + Cl(t)f(t), -co fía <t <b S oo, where B is the real 2« x 2« matrix

On the length of gaps in the essential spectrum of a generalised Dirac operator