Ten new compounds having 3-[(2-naphthyloxy)alkyl]-2,4-pentanedione and 4-[(2-naphthyloxy)alkyl]-3,5-dimethylisoxazole structures were synthesized and their cytotoxicities and antiviral activities investigated. 3-[6-(2-Naphthyloxy)hexyl]-2,4-pentandione possessing inhibitory activity on the replication of both adenovirus type 5 and poliovirus type 1 at 20 micrograms/ml was the most active compound in the series.