Accurate colony size measurement in soft-bodied sessile aquatic invertebrates is more difficult than in hard corals because of the variable state of the hydroskeleton in the former. The present study examined variation in colony height, oral disc diameter and basal circumference in three species of soft coral of different morphological types (Sarcophyton elegans, Sinularia flexibilis and Dendronephthya sp.) over a 24-h period. Individual colonies changed considerably in size over this period. Coefficients of variation for height measurements and oral disc were 0.09-0.36 and 0.08-0.28, respectively, but were only 0.02-0.09 for basal circumference, in all three species. Measurements of basal circumference in the field showed the highest correlation with colony biomass (volume after water displacement in formalin) confirming basal circumference to be a sound measure of colony size in repeated measurement studies.