McIntosh apples which have been transformed to express an endochitinase gene from Trichoderma harzianum have demonstrated substantial resistance to apple scab in previous greenhouse trials. We inoculated transgenic and control McIntosh trees with ascosporic and conidial inoculum, and recorded disease development and plant growth in a greenhouse and orchard study. In greenhouse evaluations, all transgenic lines developed statistically equivalent levels of scab when compared to the McIntosh control with both inoculum sources. Transgenic trees in the orchard generally had fewer leaves per tree, were shorter, smaller in diameter, and had fewer side branches than the nontransformed control McIntosh early in the growing season. However, this was primarily a reflection of different tree sizes among the transgenic lines and control trees when planted in 1999. With only one exception (TM961), current-season shoot length was not significantly different among the transgenic lines and the control McIntosh. In the orchard, the incidence of scab infection (percentage of leaves infected) on all four transgenic lines (including the vector control) was significantly equivalent to that recorded on the untransformed control McIntosh. However, disease severity (percentage of the leaf surface colonized) was significantly lower on all the transgenic lines.