Alam, M. Z., Braun, G., Norrie, J. and Hodges, D. M. 2013. Effect of Ascophyllum extract application on plant growth, fruit yield and soil microbial communities of strawberry. Can. J. Plant Sci. 93: 23-36. Plant growth and associated soil microbials were examined in several strawberry cultivars following treatment with extracts from the marine algae Ascophyllum nodosum [soluble Ascophyllum extract powder (SAEP)]. Greenhouse and field experiments were established over plots of Albion, Camarosa, Chandler, and Festival strawberries from 2009 to 2011. Soluble Ascophyllum extract powder was applied once or twice per week, or once per 2 wk at rates of 0 (control), 1, 2 or 4 g L^-l over approximately 8 wk. Subsequent rooting studies examined weekly applications of SAEP at 0, 0.2, 0.4, 1 or 2 g L^-1. Results indicate that maximum plant and berry productivities were found at 1 and 2 g SAEP L^-1 in both field and greenhouse. Chandler was the cultivar most responsive to SAEP application, while Albion was the least responsive. Soluble Ascophyllum extract powder increased colony counts in greenhouse and field soil samples with maximum colony counts at 4 g L^-1 SAEP in the greenhouse, and 1 and 2 g L^-1 SAEP in the field. Metabolic activities of soil microbes were found to increase following SAEP applications. Using the Biolog microbial analysis system, maximum average well colour development (AWCD), substrate diversity (H), substrate evenness (E), and substrate richness (S) responses were found at 4 g L^-1 SAEP in the greenhouse. However, in field trials, AWCD, H, E, and S responses to extract treatment showed successive increases at 1 and 2 g L^-1 SAEP, but reduced effect at 4 g L^-1. Soluble Ascophyllum extract powder treatment showed highest respiration rates between 0.10 and 0.40 g per week per plant while in vitro soil treatments with 4 g L^-1 SAEP reduced microbial respiration. This study suggests that SAEP applications increased strawberry root and shoot growth, berry yield and rhizosphere microbial diversity and physiological activity.