Symbiotic algae (zooxanthellae) from the mantles of fast- and slow-growing Tridacna gigas, from T. maxima, T. crocea and Hippopus hippopus were isolated and supplied to larvae of T. gigas grown under hatchery conditions. Significant differences in growth rates of the larvae and juveniles were seen between these various treatments. The larvae and juveniles which had been supplied with zooxanthellae taken from fast-growing T. gigas grew faster than those supplied with zooxanthellae taken from slow-growers. No preference was noted inT. gigas for zooxanthellae from a conspecific source, and those given zooxanthellae from T. maxima, T. crocea and H. hippopus survived equally well for the duration of the experiment, 90 days. T. gigas juveniles were able to continue to take up zooxanthellae form the environment throughout the first 38 days of their lives. Freshly-isolated zooxanthellae taken from clams which are known to be fast-growers are therefore recommended for routine use in giant clam hatchery operations.