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Changes in abundance, dimensions, and biovolume of manne microorganisms are important parameters used in the estimation of a wide range of biological processes, including cell activity, growth rate, predation, and nutrient cycling. Routine measurement of these parameters has been hampered by the tedium of counting and measuring large numbers of microorganisms, the subjechvity of visual measurement using the eyepiece micrometer, cell shrinkage during preparahon for electron microscopy, and the small size of marine bacteria and some protists. Image-analysed microscopy overcomes these problems, but extant equipment and programs are cumbersome. MacImage, an image-analysis system that interfaces an Artek 810V image-analysis computer and a Macintosh computer, has been developed using the language Forth to improve operator-machine control and versatility. By using a combination of a more sensitive camera tube, additional magnification lenses. and a software system for the Macintosh computer specifically tailored for the measurement of marine microorganisms, we are now able to accurately and rapidly determine the abundance, size, shape, volume, and surface area of microorganisms ranging from marine bactena to fish larvae. Data collected by this system can be transferred directly to other Macintosh spreadsheet and graphics programs. To dustrate the versawity of MacImage, we present data on size, biovolume, biosurface area, and abundance of a natural sample of picoplankton from Narragansett Bay, Rhode Island; size and biovolume data on a cultured clone of Synechococcus; and size, shape and area on the larvae of the winter flounder Pseudopleuronectes amencanus.