The taxonomic status of the fungal genus <i>Endothia</i> was recently changed in a controversial monograph by Barr (Barr, M. E. 1978. The Diaporthales of North America. Mycol. Mem. 7. J. C. Cramer. 232 p.), who divided the genus into two separate genera, <i>Endothia</i> and <i>Cryphonectria</i>, based on differences in ascospore shape and septation, stromatic configuration and distribution of stromatic tissues. This group of fungi traditionally contains some important plant pathogens; its taxonomic position needs to be resolved. The morphological criteria used by Barr were reinvestigated. Polyacrylamide gel electrophoresis and fungicide sensitivity assays were also used to examine biochemical relationships among the organisms and to establish additional means of distinguishing among the closely related taxa. The morphological features of 12 species of <i>Endothia</i> were examined. Those species with 2-celled, ovoid ascospores produced valsoid stromata, while organisms associated with nonseptate, allantoid ascospores formed diatrypoid stromata. Pseudoparenchymatous tissue was observed along the edge of the stroma, while prosenchyma was confined to the stromatic center. The major criteria used by Barr were confirmed. Polyacrylamide gel electrophoresis was used to separate the buffer-soluble proteins of 78 isolates, representing 13 species of <i>Endothia</i> and <i>Cryphonectria cubensis</i>. Intraspecific variation of banding patterns was less than interspecific differences; the species were separated by this technique. The banding patterns of <i>E. eugeniae</i> isolates closely resembled those of <i>C. cubensis</i>: these organisms may be conspecific. Hypovirulent isolates of <i>E. parasitica</i> could not be distinguished from wild isolates. The banding patterns of specific isozymes were species specific; the use of isozyme analysis has great potential for future taxonomic and genetic studies. The sensitivities of <i>E. parasitica</i> and <i>E. gyrosa</i> were determined for 23 different fungitoxicants. The two species were differentially sensitive to cycloheximide, with ED₅₀ values of 0.01-0.03μg/ml and 1.0-2.0 μg/ml for <i>E. gyros</i> and <i>E. parasitica</i> respectively. Differential sensitivities were not exhibited toward the remaining fungitoxicants; these fungi probably share many biochemical processes and response mechanisms. Barr’s classification system is technically correct and it seems to organize relationships within the entire order in a uniform manner. Its adoption is recommended with some hesitation since the influence of host on stromal development is not fully understood. / Ph. D.