Heterotrophically or photomixotrophically initiated callus cultures of Citrus paradisi, C. limon and C.aurantifolia were grown on different nutrient media and under different light regimes. Calli of C.paradisi that contained > 140 mg chlorophyll per kg wet weight accumulated about 40 volatile mono- and sesquiterpene hydrocarbons, oxigenated terpenes and aliphatic aldehydes. Upon five subcultivations the best yielding callus contained about 5% (186 mg × kg-1 wet wt) of the volatiles found in peel tissue (exo/mesocarp section), and about the twentyfold amount of that found in the fleshy endocarp. The composition of the essential oils from most of the cell cultures equalled grapefruit peel oil, but was shifted to a more fruit flesh-like composition, after the concentration of gellan gum in the medium was increased from 3 to 9 g per L. C. limon produced 11 monoterpenes and n-nonanal (40 mg × kg-1 wet wt max.), and C.aurantifolia yielded limonene only (4.4 mg × kg-1 wet wt max.). For all of the indicated species chlorophyll content and accumulation of volatiles were positively correlated. Addition of exogenous valencene to suspended cells of C.paradisi led to a stable concentration of the conversion product nootkatone. This stably maintained level suggested that a decreased catabolism of available carbon sources might have accounted for the significant accumulation of essential oil constituents.