'Rainbow' (RB), a transgenic papaya cultivar hemizygous for the papaya ringspot virus (PRSV) coat protein (CP) gene, exhibits early plant susceptibility (>70% of plants infected) to mechanical inoculation with crude preparations of PRSV isolates from Hawaii. However, RB plants become highly resistant by approximately 9 weeks after seeding in the greenhouse (2.5% of plants infected) and by 13 weeks in the field (<16% of plants infected). 'SunUp' (SU), a transgenic papaya cultivar homozygous for the CP gene, exhibits complete resistance against all isolates of PRSV from Hawaii, but is susceptible to isolates from outside of Hawaii. The current research was conducted to measure PRSV-CP expression levels in RB and SU plants at comparable stages of development, and to determine if transgene expression levels correlate with resistance. In all experiments, older RB and SU plants had lower average CP expression levels than younger seedlings, while RB had higher average CP expression levels than equivalently aged SU. For RB plants, CP expression level at inoculation was positively correlated with subsequent symptom expression in 3 out of 4 greenhouse trials and with percent infection in both field trials. Differences in CP transgene expression levels and early plant susceptibility of RB plants demonstrate that careful attention must be focused on plant development when evaluating transgenic plants for desired characteristics.