A growing number of studies have been showing that dietary probiotics can exert beneficial health effects in both humans and animals. We previously demonstrated that dietary supplementation with Lactobacillus rhamnosus - a component of the human gut microflora - enhances reproduction, larval development, and the biomineralization process in Danio rerio (zebrafish). The aim of this study was to identify the pathways affected by L. rhamnosus during zebrafish larval development. Our morphological and histochemical findings show that L. rhamnosus accelerates bone deposition through stimulation of the expression of key genes involved in ossification, e.g. runt-related transcription factor 2 (runx2), Sp7 transcription factor (sp7), matrix Gla protein (mgp), and bone gamma-carboxyglutamate (gla) protein (bglap) as well as through inhibition of sclerostin (sost), a bone formation inhibitor. Western blot analysis of mitogen-activated protein kinase 1 and 3-(Mapk1 and Mapk3), which are involved in osteoblast and osteocyte differentiation, documented an increase in Mapk1 16 days post fertilization (dpf) and of Mapk3 23 dpf in individuals receiving L. rhamnosus supplementation. Interestingly, a reduction of sost detected in the same individuals suggests that the probiotic may help treat bone disorders.