ABSTRACT: In order to understand the phenomenon of heterozygote deficiency (F_is) in marine molluscs, all the relevant literature concerning this phenomenon in the Mytilus edulis species complex was reviewed. Due to large heterogeneity inmethods of data analysis, in particular for the choice of the estimator measuring heterozygote deficiency and for the choice of the testing procedure, no clear overview was possible. To overcome this problem, similar estimators and tests were used toanalyze or re-analyze some additional large data sets from the Baltic (M. trossulus), France (M. galloprovincialis), England (M. edulis), Atlantic USA (M. edulis) and Pacific USA (M. trossulus). Large and significantheterozygote deficiencies exist in these data sets. Estimated F_is (F-hat_is) values are generally higher for Lap than for other loci, consistent with an effect of selection. No other significant variation ofF-hat_is values across loci within each data set or across data sets for each locus was detected, however no specific test has been designed for this null hypothesis. The possible contribution of a Wahlund effect to explain theheterozygote deficiency is discussed. It is likely that there is no unique explanation of heterozygote deficiency (such as a Wahlund effect or selection) in M. edulis or in other organisms, and that species-specific, locus-specific orpopulation-specific explanations are to be sought.