Previous research has shown that gut-bacterial populations in farmed abalone are shaped by ontogeny, their macroalgal diet substrates, bacteria present in the environment, and other environmental factors. It has been suggested that weaning postsettlement diatom-fed abalone onto artificial feed may alter the natural succession of bacterial establishment in their guts. The gut-bacterial composition of Haliotis midae (5–9 mm) weaned onto either a commercial formulated feed (Abfeed-S34), fresh kelp (Ecklonia maxima), or a commercial formulated feed containing kelp (E. maxima) (Abfeed-S34K) was compared. The gut microbiota of postsettlement diatom-fed abalone was dominated by Planctomycetes. Weaning to kelp or formulated feed resulted in a significant difference in the gut-bacterial communities, with the Firmicutes becoming dominant in the kelp-fed treatment and the Proteobacteria dominating in the two formulated feed treatments (analysis of similarities: R = 0.77, P = 0.02). The gut microbiota of kelp-fed abalone displayed a relatively low diversity compared with that of abalone fed formulated feeds and was dominated by fermentative Clostridia bacteria. There was no difference in gut-bacterial communities of the abalone fed commercial formulated feed with or without kelp inclusion. The dominance of the Clostridia in the kelp-fed abalone guts is explained by the large volume of fermentable polysaccharides present in macroalgae, whereas the energy-rich formulated feeds appeared to reduce the selectivity of the gut environment resulting in a relatively higher bacterial diversity.