Emerald ash borer (EAB), a phloem-feeding beetle native to Asia, was first discovered in southeastern Michigan and Essex County, Ontario in June 2002. Little information about EAB biology or control options was available from Asia. Initial studies indicated that the EAB life cycle was univoltine. Adults oviposited during the summer, larvae fed from late summer to fall, then overwintered as pre-pupal larvae, pupated in spring and emerged as adults the following summer. Our recent observations, however, indicate that at least some larvae feed for two summers. Such prolonged development, if common, would strongly influence EAB spread, population dynamics and survey activities of program managers. We evaluated EAB larval development in ash trees with varying EAB densities from 2003-2005. Relative proportions of early and late instar larvae and prepupae were determined by debarking ash trees in localized outlier sites and in severely impacted sites in southeastern Michigan. Preliminary results indicate prolonged development is more common in trees with low-density EAB populations, while most larvae in heavily infested trees develop in a single year.