The present paper reports for the first time the transformation of an organic selenium compound into red selenium (Se), which causes the intense red pigmentation of Lentinula edodes (shiitake mushroom) mycelia. The biotransformation of 1,5-diphenyl-3-selenopentanedione-1,5 (diacetophenonyl selenide, preparation DAPS-25) was studied in liquid- and solid-phase cultures of L. edodes. In liquid culture medium, a red color develops in the mycelium at initial DAPS-25 concentrations equal to or higher than 0.1 mmol/l. The intensity and initiation time of coloration is Se concentration-dependent. Semiquantitative data obtained by physicochemical methods on the extent of Se and acetophenone production suggest that L. edodes is able to absorb and/or destruct this organic Se xenobiotic.