Five optically active and sixteen racemic lactones (nine of them new) of bicyclo[4.3.0]nonane structure were synthesized. IC(50) values for the following phytopathogens were determined: Aspergillus ochraceus AM 456, Fusarium culmorum AM 282, Fusarium oxysporum AM 13, Fusarium tricinctum AM 16. Effect of compound structures, especially stereogenic centers, on fungistatic activity has been discussed. The highest fungistatic activity was observed for trans-7,8-dibromo-cis-3-oxabicyclo[4.3.0]nonan-2-one (3c), IC(50) = 30.1 μg/mL (0.10 μM/mL), and cis-7,8-epoxy-cis-3-oxabicyclo[4.3.0]nonan-2-one (3b), IC(50) = 72.2 μg/mL (0.47 μM/mL), toward F. oxysporum AM 13.