In 1992, a pycnidial fungus isolated from diseased amaranth plants was found to be the causal agent of the observed stem and leaf blight. Inoculations performed in the greenhouse revealed that this organism caused a disease characterized by a general blight. The isolate was tentatively identified as a member of the genus Phomopsis and its morphological characteristics were compared to those reported in the literature for Phomopsis species collected from amaranth. The Florida isolate, ATCC 74226, was found to produce extremely large alpha conidia. Beta conidia were produced, as well as a third type of conidium of intermediate shape. Phylogenetic analysis, based on the sequences within the internal transcribed spacer regions of the ribosomal DNA, revealed that isolate ATCC 74226 differed substantially from the other Phomopsis species tested, including those that are known to produce the third type of conidium. Based on the morphological characteristics and DNA sequence information the isolate is described here as a new species of Phomopsis.