Exposure experiments in medaka and photodecomposition tests were performed using a metabolite of bisphenol A [4-methyl-2,4-bis( p -hydroxyphenyl)-pent-1-ene; MBP], the solubility limit of which is 42 mg/l of water. Three adult medaka were kept in a 2 l glass beaker at 25 ± 1°C for 4 days. The LC₅₀ for 96 hr was > 1000 ppb. The measured average MBP concentration in the breeding water (nominal concentration of 100 ppb) was 49.2 ppb. The average concentration in the whole bodies of medaka after 4 days was 1.92 mg/g-wet body, and the bioconcentration factor (BCF) of MBP was calculated to be 39.0. MBP in water and acetone was decomposed very easily, with about 98% of the MBP being decomposed after several hours under sunlight. MBP was also decomposed after 48 hr of illumination under a white fluorescent lamp.