An optical comparator system has been constructed which measures the diameter of bacterial zones on agar plates and stores the data in a computer. The optical system provides a method of superimposing and adjusting the image of a circle drawn on a CRT over the image of the bacterial zone until a match is made. An analogue/digital converter and single-board computer scale and present the diameter on a digital display. The apparatus provides an alternative method of measuring bacterial zones that is more accurate and faster to use than a Vernier calliper and less costly than an image analyser.