Isolation and characterization of Halothiobacillus sp. from the shrimp aquaculture farm soil and their sulphur oxidation ability and utilization of H S in in-vitro model. 2 Starkeys mineral salt medium was used to screen autotrophic sulphur oxidizing bacteria. For the qualitative screening, bacterial isolates were inoculated in mineral salt medium containing bromo phenol blue indicator to monitor change in pH. The isolates were studied further for their sulphate ion production, sulphur oxidase enzyme production and utilization of Na S. Identification was carried out by 2 conventional biochemical and molecular methods. Fifty isolates showed distinct sulphur oxidizing ability on the mineral salt medium. The pH reduction test revealed that out of fifty isolates six could efficiently reduce the pH of the medium to 3.0 from an o-1 initial pH of 7 within 96 hr of incubation at 30 C. Maximum sulphate ion (12.65 mg ml) and sulphur oxidase-1-1 enzyme (16.64 mM sulphate hr ml) was produced by a bacterial isolate, Halothiobacillus sp. strain rk3. All the six isolates efficiently utilized Na S in in-vitro conditions. Conventional and molecular identification (16S 2 rRNA sequence analysis) revealed that the sulphur oxidizing bacterial isolates belonged to Halothiobacillus spp. Furthermore, sequencing similarity calculation showed an average nucleotide identity (ANI) values higher than 99% which suggests that the isolates were not genetically different. The present investigation revealed the presence of Halothiobacillus sp. as natural microflora of farm soils in shrimp aquaculture.