A half-diallel design, with parents, involving a random sample of nine partially inbred (S1) lines was used to examine the genetic control of bolting in a broad-based North-West European sugarbeet breeding population. The diallel was analysed using the Hayman (1954a) and Jinks & Hayman (1953) analyses. Additive genetic effects were shown to be highly important and significant dominance effects were found. A general trend for bolting resistance to be dominant was detected but there was also evidence that the control was ambidirectional. Furthermore, the dominance effects seemed only partially effective. Some indication of epistatic effects was found, although the possibility that this was spurious, caused by failures in the genetic assumptions required by the analyses, is discussed. The genetic control of bolting in the breeding population appeared complex.