ABSTRACT: In symbiosis, 2 taxonomically different organisms co-exist, each pursuing their own agenda and yet, they are linked in one entity. A mutualistic symbiosis may break up if it is no longer beneficial to either one of the partners. Changing needsover time or changing environmental conditions may prompt symbiont switching. For example, corals may survive elevated temperatures by switching their algal symbionts. If switching occurs, the new combination of host and symbiont genotypes may performbetter. Conversely, the partners may be fixed for life, with the degree to which the mutualism responds to changing selection pressures dictated by the existing partners. Understanding the genotypic dynamics of a mutualism is important for predicting thepotential resilience of a mutualism over time and in the face of environmental perturbations. Although mutualisms tend to be characterized at the species level or higher, host-symbiont dynamics is an individual-level question, requiring individual-levelanalysis. We used multilocus DNA fingerprinting to examine long-term temporal and spatial symbiont change in the mutualism between the octocoral Plexaura kuna and its algal symbionts (zooxanthellae). We monitored zooxanthella genotypes within acolony for up to 10 yr, among P. kuna clonemates, across different habitats and in colonies transplanted to novel environments. In all instances, the prominent zooxanthella genotype within a P. kuna colony remained unchanged althoughzooxanthella genotypes varied among genetically distinct P. kuna colonies. Such tremendous temporal and spatial stability may occur in other coral hosts, influencing the reaction and survival of mutualisms during environmental change.