A series of aryloxy alkyl diketones II was synthesized and screened in vitro for antiviral activity. The effect of various substituents on the phenyl ring and the length of the alkyl bridge was examined to establish the requirements for optimum activity. One of the most active members of the series, 4-[6-(2-chloro-4-methoxy)phenoxy]hexyl-3,5-heptanedione, exhibited a high level of activity against DNA and RNA viruses in both the tissue culture and organ culture screens and was particularly effective against herpesvirus types 1 and 2.