Isolates of Peronospora tabacina , the tobacco blue mold pathogen, were obtained from naturally infected plants growing in the field and were identified by location, year and fungicide treatment of the plant. A quantity of sporangiospores from an isolate was used to prepare a fatty acid profile characteristic of the isolate. Sporangiospores for profile preparation were harvested directly from field-grown plants or from infected plants produced by inoculation and incubation in the laboratoy. Care was used to maintain the integrity or purity of the isolates. To prepare fatty acid profiles, sporangiospores (spores) were washed from leaves with water applied with a hand sprayer. The aqueous spore suspension was filtered through four layers of cheesecloth and centrifuged @ 8000 rpm in a table top centrifuge to from a pellet of spores. The pellet of spores, usually 0:2-0.4 cc in volume, was washed by suspension in 10 cc of water, followed by centrifugation. Water was added to a volume of 0.2-0.4 cc of spore pellet to make a final spore suspension of 1cc. This 1 cc of spore suspension was subjected to a standardized process consisting of saponification, methylation, extraction, washing and injection into an automated gas chromatography system designed to separate, quantify and identify fatty acid methyl esters. This is the standard procedure used in the Microbial Identification System for identification of microorganisms. This system is available commercially. The time required from the harvest of spores to the printing of a fatty acid profile was about four hours on average. The fatty acid profiles of the various isolates we have prepared will be presented and compared. This will include profiles from isolates that were resistant to concentrations of 100 micrograms of metalaxyl per cc and isolates that were sensitive to metalaxyl.