A 24-year-old woman had fatal pneumonia-associated adult respiratory distress syndrome caused by Rhodococcus species. Histological examination of lung biopsy tissue showed intracellular coccobacillary microorganisms. Antimicrobial susceptibility tests on the patient's blood isolate showed that it was resistant to clindamycin and norfloxacin but susceptible to several other antimicrobial agents. Also, the isolate's biochemical reactions and pattern of RNA gene-containing restriction fragments were significantly different from those of the 20 recognized Rhodococcus species, suggesting that this patient's infection was caused by an as yet uncharacterized Rhodococcus species. Of the 17 human cases of nonequi Rhodococcus species infection reported to date (including the current case), nine patients were immunocompetent, five had disseminated infection, and four died. Further studies will be required to unequivocally establish the species status of this patient's Rhodococcus isolate biochemically and genetically.