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Light and scanning electron microscopy were used to describe Neidium petersiveri sp. nov., a previously unnamed diatom found in the Canadian waters of Lake Superior. It had been reported previously as single cells of an unidentified Neidium species from four widely separated regions in North America: Michipicoten Island (Lake Superior, Ontario, Canada), Cape Cod (Massachusetts, United States), a Quebec river (Quebec, Canada), and the Back River (Nunavut, Canada). There are superficial resemblances to Neidium mirum Krasske and Neidium calvum Østrup, but there are significant differences in frustule size and shape as well as the pattern of striae on the valve face that render N. petersiveri unique. There was not a significant correlation between frustule lengths and widths in N. petersiveri, owing to a relatively small range within the width data that resulted in much higher width-to-length ratios among shorter cells than for longer cells (74 cells measured). Other significant aspects of morphologic variation include various shapes and configurations of the lacinial fissures and of the proximal raphe fissures, and the sporadic occurrence of pit-like depressions in the axial and central regions of the valves, which suggest that variations in these characters likely have limited value as discriminating characters at the species or subspecies levels. [ABSTRACT FROM AUTHOR]