Sixteen highly branched (colonial) mutants of Fusarium graminearum A3/5 were isolated at the end of 11 Quorn® myco-protein production fermentations. These ranged from the highly branched C134-3 to the sparsely branched C145, which was similar to A3/5 in liquid culture. Although allelic complementation was observed to occur between some of the mutants, heterokaryon analysis revealed that all the highly branched mutants belonged to a single complementation group. Mixed cultures of A3/5 and six of the colonial mutants were grown in glucose-, ammonium-, magnesium- and sulphate-limited chemostat culture. One mutant (C153) had a selective advantage over A3/5 in all nutrient limitations tested; four mutants (C134-1, C134-3, C137-1, and C135) had a selective advantage over A3/5 in all except one nutrient limitation; and one mutant (C139) had a selective advantage over A3/5 in magnesium-limited chemostat culture only. Four mutants (C134-3, C139-1, C153 and C135) had a selective advantage over A3/5 when the dilution rate was increased above the critical dilution rate (0·22 h−1) and nutrients were present in excess. These results suggest that the growth conditions in the Quorn® production plant (which is operated as a glucose-stat) favour mutations that confer general growth rate advantages, but these mutations may result in growth disadvantages when nutrient limitations are imposed. As all of the mutations appear to occur in one gene or gene cluster, the differing patterns of selective advantage/disadvantage for the six mutants studied suggests that regulatory genes may also be involved or that different sites of mutation can lead to a variety of pleiotropic effects.