Despite the economic importance of eel captures in the Mar Menor, a hypersaline coastal lagoon, few studies have been conducted in relation to the biology and ecology of this specie. The aimed of this study was to describe the relationship between some biological and morphological parameters and the silvering process of the European eel in this lagoon. A total of 290 yellow and silver eels were anaesthetized and the following measures were obtained: total length (TL, mm), total weight (gr), horizontal and vertical eyes diameters, pectoral fin length. Ocular (OI) and fin (FI) indexes as well as scaled mass index (SMI) were calculated. Additionally, 63 eels were euthanized with an overdose of anesthetic and gonads were dissected, weighted, fixed in Bouin’s fixative and routinely processed for histology. The gonadosomatic index (GI) was calculated. Sex was identified macroscopically and microscopically. Results indicated that Mar Menor eel population is dominated by females which may be due to local conditions, such as the density as it was previously reported in eels from other environments. A significant GSI increase was detected with the silvering process. Although silver eel average size (63.9 cm, range 37-83.5) was smaller than data reported in eels from other lagoons, the GSI increment starts at the same size, around 50 cm TL, as previously reported. Due to the marked overlap in TL of yellow and silver eels between 300 and 400 gr, it was difficult to establish the starting size of the silvering process. A decrease of the SMI with the silvering process was detected. FI showed significant differences in relation to silvering in specimens around 300 gr, while no significant differences in OI were observed (OI). Implications in eel management in the lagoon are discussed.