Single‐spore cultures of wheat‐ and barley‐adapted isolates ofSeptoria nodorumwere serially passaged through detached leaves of wheat or barley. Rare instances of change in host adaptation were observed after passaging but associated changes in other characters, including heterokaryon compatibility, identified the changed isolates as contaminants. Inoculation of progressively more dilute spore suspensions to detached leaves revealed that some isolates were highly infective to the host to which they were adapted, with a high probability of a single spore initiating infection. Inocula deliberately contaminated with a small proportion of spores of opposite host adaptation revealed changes when passaged on wheat that mimicked the rare changes observed on passaging pure wheat‐ or barley‐adapted isolates. These results suggest that adaptation to wheat or barley is a stable property of pure cultures ofS. nodorumand that the occasional changes in host adaptation observed on passaging result from selection of con
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