Aims To investigate effects of fluoride ion (F-) on, and kinetics of its movement into and out of, spores and growing cells of Bacillus species. Methods and Results Effects of F- on Bacillus cell growth, spore germination and outgrowth and heat resistance were investigated, as well as F- movement into and out of spores using F-19-NMR. F- inhibited Bacillus subtilis spore germination and outgrowth, and YhdU, now named FluC, was crucial to prevent F- accumulation in growing cells and to minimize F- inhibition of spore germination. Dormant wild-type, yhdU and coat defective B. subtilis spores, and Bacillus cereus spores incubated in 40 mmol l(-1) NaF for 48 h accumulated 2-2.6 mol l(-1) F- and its movement into Bacillus spores was highest at low pH. Bacillus subtilis spores lacking Ca-dipicolinic acid accumulated higher F- levels than wild-type spores. Conclusions These results are consistent with F- incorporation into the dormant spore core, and as HF and/or NaF, but not CaF2. YhdU played no significant role in F- uptake or efflux in dormant spores, but assisted in F- export early in spore germination. Significance and Impact of Study This knowledge provides new insight into effects of F- on Bacillus cells and spores and how this anion moves into, and out of spores.