Department of Chemistry, University of Roorkee, Roorkee-247 672 Department of Chemistry, Kurukshetra University, Kurukshetra-132 119 Manuscript received 15 February 1982, accepted 10 December 1982 Sixteen new 1-(2-benzothiazolyI)-5-aryl-3-methylpyrazoles (1) have been synthesised by the condensation of 1-aryl-1,3-butanediones and 2 hydrazinobenzothia­zoles. Bromination study of I(R=R'=H) in Br2/AeOH has revealed that electro­philic attack of bromine occurs in pyrazole ring at position 4. The mass spectral studies of these compounds involve fragmentation via three principal modes (a) fission of benzothiazole ring, (b) fission of pyrazole ring resulting in the loss of 2-benzothia­zolyldiazonium radical and (c) fission of pyrazole ring with concomitant loss of CH2CN from the molecular ion.