CIS-1 is a particle sizer based on time-of-transition principle. It scans particles with a laser beam, and determines their sizes by measuring the time lengths of light obscuration by the particles. Studies have shown that CIS-l provides good size information for standard reference spherical particles and for homogeneous material. This study examines the accuracy of its size information by using field particles obtained in the ocean. These particles differ from the above materials in that field particles have a wide range of shape and optical properties; they may affect the performance of a particle sizer. The volume-based cumulative size distribution of primary particles in 8 offshore and 4 coastal water samples were obtained by CIS-l and by microscopy. Their comparison generally showed good agreements. The maximum difference in the median size of the same sample was 1 µm, or 17%, and the standard deviations and the modes also were close. The average relative size deviations between the two methods varied between 2% and 17.3%. The mean of the average relative deviation among all samples was 9.7 %. The results of this study also showed that the distributions of primary particles in all samples were highly similar, irrespective whether they were from offshore or coastal waters. There were no primary particles larger than 16 µm, and their median sizes were within the range of 4-6 µm. By averaging the size distributions obtained for all samples, the median size and the mode of the resulting distribution were 4.7 and 5.5 µm, respectively.