Abstract: A set of N,N'-diarylalkanediamides was synthesised. The compounds were tested for their antimycobacterial and antialgal activity. The antimycobacterial activity of N,N'-diarylalkanediamides depends on the lipophilicity of the respective acid. Antimycobacterially active substances were found only in the series of N,N'-diarylethanediamides and N,N'-diarylbutanediamides. Other compounds (derivatives of pentane-, hexane-, octane- and nonanediamide) were inactive against various strains of mycobacteria. The compounds inhibited growth and chlorophyll production in Chlorella vulgaris. Their relatively low antialgal activity is caused by their low aqueous solubility, and hence by a restricted passage of the inhibitor through the hydrophilic regions of thylakoid membranes.