A common molecular size and shape was found among the attractive compounds when 124 alkyl carbonyl and few other types of compounds were studied for ovipositional attractancy of mosquitoes, Culex pipiens quinquefasciatus Say. The attractive compounds were mostly in 9-atom-length and branched. A methyl branch on α carbon or an ethyl branch on γ carbon from the functional group furnished attractancy, but those on β or δ carbons did inactivity with no exception. A methyl branch on ω-1 carbon furnished the best attractancy. Regardless the types of compounds; acids, ketones or esters, those in the similar contour showed the similar levels of attractancy. The 11-or more-atom-long compounds tended to be extremely repellent. All alcohols were inactive with one exception. The compounds in beyond 24-atom-length were inactive because of the involatility.