The purpose of this study was to determine the inhibition of chitosan from crab cultivation waste "soka" (Scylla sp) against histamine-forming bacteria in tuna (Euthynnus affinis). The formation of histamine on the cob was caused by bacteria that could produce the enzyme histidine decarboxylase to transform histidine into histamine. Several studies had described the ability of chitosan to inhibit the activity of histamine-forming bacteria. In this research, There were three stages. The first stage was characterization of chitosan from waste shell of cultivated "soka" crab. The second was isolation of bacteria producing histamine on the cob and the third was to test the activity of chitosan against bacteria producing histamine. Chitosan from waste shell of cultivated "soka" crab contained 10.07% water, 0.20 % fat, 1.42 % ash content, 3.18% protein, and 58.99 % degree of deacetylation. The isolation of bacterial symbionts on the cob re-examined in the formation of histamine thus obtained 4 isolates; that were Serratia marcescens, Enterobacteriaceae sp, Enterobacter gergoviae and Citrobacter amalonaticus. Testing of chitosan inhibition against bacteria Serratia marcescens, Enterobacteriaceae sp, Enterobacter gergoviae and Citrobacter amalonaticus obtained a clear zone, respectively, 0.54 ± 0.03 cm; 0.59 ± 0.01 cm; 0.41 ± 0.01 cm; and 0.40 ± 0.05 cm. Keywords: chitosan, histamine, cob, bacteria, isolate