An experiment was conducted to evaluate the effects of a yeast culture/enzymatically hydrolyzed yeast product (Celmanax; Varied Industries Corp. Inc., Mason City, IA) and a diet type on performance, nutrient digestibility, intestinal morphology, carcass composition and visceral organ mass by guinea pigs. Eighty-eight guinea pigs (initial BW = 453.21 g ± 21.90 g) were group fed in 24 pens during a 69 d feeding period. Treatments were arranged in a 2 × 2 factorial and included a concentrate diet (C) or a mixture of forage and concentrate diet (M) and supplementation of Celmanax (Ce, 0 or 0.1% of diet DM). No type of diet × addition of Ce interactions were detected in any variable measured (P ≥ 0.157). Animals fed the M treatment had greater CMS, ADG, and improved G:F during the feeding period (P < 0.001). Supplementation of Ce had greater CMS and improved G:F during the first 49 d of the experiment (P ≤ 0.048). However, overall performance was not altered by Ce supplementation (P ≥ 0.247). Fat digestibility was greater for C (P < 0.001), and crude fiber digestibility was decreased (P = 0.004) compared with M. Nutrient digestibilities were not altered by Ce supplementation (P ≥ 0.285). Feeding M increased dressing percent, empty body weight, fractional mass of: liver, large intestine, portal drained viscera, and total splanchnic tissue (P ≤ 0.019) compared with C. Carcass characteristics and visceral organ mas were not altered by Ce supplementation. Fatty acid profile from adipose tissue was not different across treatments (P ≥ 0.053). The relation between n-3:n-6 fatty acids was greater for animal fed M compared with C (P = 0.019). Villus height and crypt depth were increased by Ce supplementation (P ≤ 0.019). Animals fed M diet had greater crypt depth (P = 0.05). Results indicate that feeding a mixture of forage and a concentrate diet results in positive effects in performance and carcass characteristics, and supplementation of Celmanax may have positive effects on performance and intestinal characteristics under both feeding regimens.