Egg clusters of the checkerspot butterfly, Euphydryas editha, are contagiously distributed among host plants (Pedicularis semibarbata) at one mid-elevation montaine site in California. Large, isolated plants receive more egg clusters than is expected from their proportional representation in the population. This study attempts to determine the behavioural causes of that contagious distribution. Non-random alighting by ovipositing females appears to be the primary cause of the observed contagious distribution. Females alight more often than would be expected by chance on large, isolated plants. Females also appear to exhibit a weak post-alighting bias in favour of large, isolated plants, though this bias is not statistically significant. However, manipulative experiments reveal that postalighting acceptance is not random; certain plants are accepted repeatedly while others are rejected repeatedly. The results also indicate that searching females distinguish among different classes of plants within a host species.