ABSTRACT.1The locust displayed clear preferences in the ranking of five possible food plants: wheat>barley =Cypeius alternifolia>chives>lemon grass.2In two‐species stands of constant numerical density, the locust grazed selectively on the less‐favoured plant when it was relatively less abundant.3The behavioural mechanism of selective grazing was a change in the readiness to eat a plant as its relative frequency changed: i.e. the consumption/encounter ratio rose as relative density fell.4With the exception ofCyperus, which nutritionally is below chives, the preference ranking matched nutritional ranking as estimated by rearing‐success on single plant species.5It is suggested that such negative frequency‐dependent selection may be a common strategy for polyphagous herbivores as it permits a wide dietary intake without hazards from potentially toxi