The propagation of viruses in a growing plaque has been measured using a digital image acquisition and analysis system. Plaques of phage T7 incubated at 37 degrees C and illuminated against a dark field emerged as dark growing spots against a background of host bacteria. Images of the growth were acquired using a charge-coupled device (CCD) camera at l-h intervals over 24 h. The first 10 h of plaque development coincided with rapid growth of the agar-immobilized Escherichia coli host, measured as a reduction in gray value. Following this period, the average radial velocity of plaque growth remained constant at 0.059 mm/h while the standard deviation about this velocity increased. These results suggest the suitability of the system for spatially resolving the dynamics of viral evolution during plaque growth.