Di-isopropylnaphthalene (DIPN) has highly persistent and bioaccumulative properties, and a large amount of DIPN is used as a PCB substitute in Japan. However, DIPN in the environment has not been thoroughly investigated. In addition, mono-isopropylnaphthalene (MIPN) and tri-isopropylnaphthalene (TIPN), which are the homologues of DIPN, have similar properties to DIPN. In this study, simultaneous analytical methods for MIPN, DIPN, and TIPN for air, environmental water, sediment, and biological samples were developed, and the resultant contamination caused by each in the environment was investigated. DIPN was detected at 1.1 ± 0.38 ng/m(3) in air and between < 1.9 and 9.8 ng/L in river water, but MIPN and TIPN were not. In Lateolabrax japonicas (Japanese sea perch), TIPN was detected from only females at between 0.65 and 1.4 ng/g-wet. DIPN was detected from all perches at between 1.2 and 3.4 ng/g-wet. DIPN and TIPN isomer fingerprints in females were different from those in the reference standard stock solution ones. In sediments, MIPN, DIPN, and TIPN were detected at between < 0.16 and 8.6 ng/g-dry, between < 1.1 and 4400 ng/g-dry, and between < 0.83 and 500 ng/g-dry, respectively. The contamination trend of DIPN in the sediments was similar to that of PCBs.