Twelve 4-hydroxycoumarins derived from the condensation of diethyl carbonate and 2-hydroxyacetophenones and forty-one dicoumarols achieved from nucleophilic substitution reaction between 4-hydroxycoumarins and interesting aldehydes. These compounds were subjected to four bioassays. For brine shrimp lethality test, dicoumarol with two tertiary butyl substituents (D37) displayed high activities. In general 4-hydroxycoumarins were found to exhibit higher activity than dicoumarols. Among 4-hydroxycoumarins, a methoxy group at C-5 (3) showed a good tendency in weed growth inhibition against Mimosa pigra Linn. In the case of acute toxicity test against Oreochromis niloticus, high activities were observed in two dicoumarols, namely, 3,3'-(methylene) bis-4-hydroxycoumarin and 3,3'-(4-nitrobenzylidene) bis-4-hydroxycoumarin. For antibacterial activity, dicoumarol with a methoxy group (D25) revealed high activity. These obtained results manifestly endorsed theconceptual ideas that the type and position of substituents affected the biological activity.