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The factors controlling the growth of the edible, red seaweed, Gracilaria parvisporaAbbott (long ogo), on the south reef of Molokai, HI, were investigated to determine where productive new plantings could be located. Experiments were conducted in October, 1997, and March and June, 1998, in which G. parvisporabiomass production was correlated with water quality factors measured at six sites over each 21-day experiment. Water motion, temperature, salinity, nitrate and phosphate varied among sites and experiments, but were not significantly (P0.05) correlated with growth. A strong correlation, however, was found between biomass production and ammonia concentration (r=0.91, P<0.001). Ammonia levels ranged from 0.2–4.0mmol m–3over sites and experiments but were skewed towards low values, as was biomass production. The transient nature of ammonia distribution on this reef explains the patchy distribution of locations at which G. parvisporais productive, noticed in previous experiments. Large-scale Gracilariaculture on such a reef would require adding an external source of fertilizer, which may disrupt the reef ecology. An alternative is to develop a dispersed form of cultivation at sites that receive ammonia enrichment from the land, in which case the crop can help absorb excess nutrients entering the reef.