Scientists investigating toxicants such as endocrine disrupting chemicals (EDCs) at the cellular at the sub-cellular level are often faced with criticisms as to how these effects can be extrapolated to the level of individuals and their populations. This report aims to provide an overview of the studies undertaken on crustacean model, Echinogammarus marinus LEACH (AMPHIPODA), and intersex phenotypes, at the individual and population levels, and provide additional emergent data at the genomic level. These, normal and intersex, males and females have been investigated by cross-hybridisation microarray analysis and specific sexually dimorphic genes and corresponding properties identified between each sexual phenotype. The morphology, physiology and histology of these intersexes have been investigated in detail and a number of reproductive costs have been identified including reduced fecundity and fertility. These costs have been incorporated into a population model and simulated over a ten-year period to ascertain how different levels of intersexuality affect the stability of populations. Based on the information gained through study of intersex models (with known endocrine dysfunction) together with the substantial quantity of historical data relating to effects of chemicals on amphipod fecundity, growth and mortality, the development of appropriate biomarkers is nearer to being assessed from the level of genes to that of the population.