Four collections of the red algae Chondrococcus hornemannii ( Lyngbye ) Schmitz afforded a range of monoterpenes. Structures of ten new metabolites were elucidated by using high- field n.m.r. spectroscopy techniques including 2.D proton-carbon correlation experiments. The new monoterpenes were: (Z)-3-bromo-8-chloro-6-chloromethyl-2-methylocta-1,6-diene (9,6-hydroxymethyl-2-methylocta-2,8-dien-6-ol (6), (Z)-6-hydroxymethyl-2-methylocta-1,6-diene- 3,8-diol (7), (2)-6-hydroxymethyl-2-methyIocta-2,6-dien-8-ol (8), (2)-8-chloro-6-chloromethyl-3- methoxyocta-1,6-diene (9), (Z)-8-chloso-6-chloromethyl-2-methylocta-l,6-dien-3-ol (10), (2E,62) 8-chloro-6-chloromethyl-1-methoxy-2-methylocta-2,6-diene (11), (2E,6Z)-8-chloro-6-chloromethyl- 2-methylocta-2,6-dien-1-ol (12), (Z)-6-chloromethyl-3,8-dimethoxy-2-methylocta-1,6-diene (13) and its positional isomer (Z)-6-chloromethyl-1,8-dimethoxy-2-methyoca-26-diene (14). Although it is possible that some of the metabolites listed may be artefacts of extraction, circumstantial evidence is offered which suggests that most isolates were present in the plant prior to extraction.