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The metalloendopeptidase AsaP1 is one of the major extracellular virulence factors of A. salmonicida subsp. achromogenes, expressed as a 37-kDa pre-pro-peptide and processed to a 19-kDa active peptide. The aim of this study was to construct mutant strains secreting an AsaP1-toxoid instead of AsaP1-wt, to study virulence of these strains and to test the potency of the AsaP1-toxoid bacterin and the recombinant AsaP1-toxoids to induce protective immunity in Arctic char. Two A. salmonicida mutants were constructed that secrete either AsaP1(E294A) or AsaP1(Y309F). The secreted AsaP1(Y309F)-toxoid had weak caseinolytic activity and was processed to the 19-kDa peptide, whereas the AsaP1(E294A)-toxoid was found as a 37-kDa pre-pro-peptide suggesting that AsaP1 is auto-catalytically processed. The LD50 of the AsaP1(Y309F)-toxoid mutant in Arctic char was significantly higher than that of the corresponding wt strain, and LD50 of the AsaP1(E294A)-toxoid mutant was comparable with that of an AsaP1-deficient strain. Bacterin based on AsaP1(Y309F)-toxoid mutant provided significant protection, comparable with that induced by a commercial polyvalent furunculosis vaccine. Detoxification of AsaP1 is very hard, expensive and time consuming. Therefore, an AsaP1-toxoid-secreting mutant is more suitable than the respective wt strain for production of fish bacterins aimed to protect against atypical furunculosis.