The effect of Alternaria brassicicola (Schw.) Wiltshire infection on the changes of nucleic acids of Brassica oleracea var. botrytis was studied. An increase in vocal nucleid adds (57%), DNA (44%) and RNA (150 %) in infected leaves was found. The increase in RNA content was due to an increase of all the four nucleotide contents. Guanylic acid increased significantly more than the other three nucleotides. The increase in G/C ratio was also due to an increase in guanylic acid. Copyright © 1985, Wiley Blackwell. All rights reserved