Fine roots are a key component of belowground carbon dynamics in forest ecosystems. However, information on fine root dynamics in mangrove forests is still limited. Therefore, in this study we examined the biomass and production rates of fine roots by using soil coring and an ingrowth core method, respectively, at soil depths of 0 cm-40 cm in Avicennia alba and Rhizophora apiculata stands in Ranong Province, southern Thailand. In these stands, the fine root biomass was ca. 3.4 kg m⁻² and 1.4 kg m⁻², respectively, while fine root production rates were ca. 450 g m⁻² year⁻¹ and 740 g m⁻² year⁻¹, respectively. Fine root biomass was not significantly different between the surface (0 cm–20 cm) and subsurface (20 cm–40 cm) soil in both stands. The fine root production rate was also similar between the soil layers in the R. apiculata stand, whereas it decreased with soil depth in the A. alba stand. The patterns of vertical distribution of fine root production rates probably reflected the species characteristics of A. alba and R. apiculata, and suggested that fine root production in the subsurface soil contributes significantly to belowground carbon dynamics, especially in R. apiculata.