Studies on the occurrence of acaricide resistance in R. (B.) microplus against organophosphate group of acaricide diazinon was conducted in target areas in 4 states of India (Uttar Pradesh, Punjab, Bihar, west Bengal). Acaricide resistance was determined by adult immersion test with mortality of the ticks and the rate of oviposition as the main criterion. The LC50, LC95, LC99.9 and discriminating dose (DD) of diazinon was determined on laboratory reared susceptible (LRS) colony of R. (B.) microplus by regression analysis and was used to assess the acaricidal sensitivity status of ticks collected from field. Acetylcholinesterase (AChE) assay with propoxur was used to determine the remaining AchE activity in susceptible and field collecteed ticks. GST activities in various ticks were assessed. Finally, the molecular basis for resistance was searched for by sequence analysis of AchE2, AchE3 and GST genes. Some novel nucleotide and amino acid substitutions were found.