Pa-som is a traditional salty fermented food made from varieties of freshwater fish with garlic and rice in Laos. A similar product, plaa-som, is made in Thailand. To investigate the fermentation process of pa-som, lactic acid production and pH were monitored during laboratory-scale production of pa-som. Furthermore, the variety of bacteria involved and their succession during fermentation were examined using culture-independent polymerase chain reaction (PCR) denaturing gel gradient electrophoresis targeting the 16S ribosomal RNA (rRNA) gene. d- and l-lactic acid production and a decrease in pH were observed in the samples after 1 day of fermentation. The lactic acid content and pH continued to increase and decrease, respectively, until 4 days of fermentation. Overall, six lactic acid bacteria and eight other indigenous bacteria species were detected during pa-som fermentation. Among the lactic acid bacteria detected, two Lactococcus and one Weissella species were detected in samples after 1-4 days of fermentation, suggesting that these three species play major roles in pa-som fermentation from the initial phase. This is the first report to investigate the fermentation process in pa-som at the microbial level. This research approach should be applicable to a wide variety of pa-som and related fermented fishery products in Laos to help understand their microbial diversity and to identify beneficial bacterial species for improving quality.