A study was performed on a population of Harris' checkerspot butterfly, Charidryas harrisii (Nymphalidae) in Princeton, Massachusetts, USA, to determine which characteristics of its host plant, flat-topped white aster (Aster umbellatus Asteraceae) a female may use to determine oviposition site. Several physical parameters of an aster population were recorded when females were laying eggs. Later, as larvae were appearing, the plants selected as oviposition sites were measured in a similar fashion. Females chose plants that were significantly taller, had more leaves, thicker stems, and were more “bushy” than the population average. Contrary to our expectations, the butterflies chose plants with fewer conspecifics than the population average. This research suggests management possibilities for the host plant population of this butterfly species.