Aims: To investigate the survival and persistence of Lactobacillus plantarum REB1 in the fermentation process of liquid pig feed on a working farm in standard production conditions. Methods and Results: Two feed types, a control diet [nonfermented liquid feed (NFLF)] and a fermented diet [fermented liquid feed (FLF)], were compared. A rifampicin-resistant mutant L. plantarum REB1-Rif(R) was used to initiate the fermentation of the feed. Inoculation with the experimental strain was repeated one or two times per week throughout the three month growing period. Four microbial groups were followed using standard microbiological techniques, as well as pH. Lactic acid bacteria (LAB) counts were high already at the beginning in FLF, while it took nine days to reach the corresponding LAB levels in NFLF. Yeasts were stable in FLF, whereas in NFLF there were occasional high counts during the first week. The numbers of Enterobacteriaceae were low, although in NFLF they were variable. The average pH in NFLF was 4.5 and 4 in FLF. Conclusions: The inoculation of L. plantarum REB1-Rif(R) provided a LAB population of log 9 colony forming units (CFU) ml(-1) from the first feeding day, stable numbers of yeast and pH, and a drastic reduction of Enterobacteriaceae. Significance and Impact of the Study: The inoculation by L. plantarum REB1-Rif(R) offered a FLF microbiologically stable from the first week in actual production conditions.