A chemical category is a group of chemicals whose toxicological properties are expected to be similar or follow a regular pattern as a result of structural similarity. The category approach is beneficial for decreasing in the resource of risk assessment for huge amount of unevaluated existing chemicals, and also in the use of all kinds of animal tests including even in vivo genotoxicity tests from a point of view of the animal welfare. The present paper reports the results of in vivo micronucleus tests of o-sec-butylphenol (CAS:89-72-5) and 2-isopropyl-5-methylphenol (CAS:89-83-8) and discusses genotoxic potential of seven alkylphenols, o-sec-butylphenol, 2-isopropyl-5-methylphenol, p-sec-butylphenol (CAS:99-71-8), 2-tert-butylphenol (CAS:88-18-6), 2, 4-di-tert-butylphenol (CAS:96-76-4), 4-tert-butylphenol (CAS:98-54-4) and 6-tert-butyl-m-cresole (CAS:88-60-8) by the category approach. Based on the negative results of in vivo micronucleus tests, it can be concluded that these category chemicals are not likely clastogenic in vivo. Further in vivo micronucleus assays on untested substances may not be required by using the category approach, but further supporting information such as physicochemical profiles and (Q) SAR predictions may be necessary to strengthen the rationale for the category approach.