Analysis of ergosterol content in different tissues of Shiitake mushrooms showed a significant difference (p < 0.01) in its distribution. Thus, the conversion of ergosterol in whole mushrooms to vitamin D2, by exposure to UV irradiation, was significantly affected (p < 0.01) by the orientation of the mushroom tissues to the UV. The highest ergosterol content was found in button mushrooms (7.80 +/- 0.35 mg/g DM) while the lowest was in enoki mushrooms (0.68 +/- 0.14 mg/g DM). The conversion of ergosterol to vitamin D2 was about four times higher when gills were exposed to UV-A irradiation than when the outer caps were exposed to the same. The lowest conversion to vitamin D2 (12.5 +/- 0.28 microgram/g DM) was observed for button mushrooms while the highest value (45.1 +/- 3.07 microgram/g DM) was observed for oyster mushrooms. The optimum moisture content of mushrooms for this conversion was around 78% on a wet basis and the temperature was around 35 degrees C.