The analysis of an infestation level of the Baltic herring (Clupea harengus L.) with Anisalfis simplex (Rud.) larvae as well as otoliths and gonad maturity examinations allowed to distinguish, among the Pomeranian Gulf and adjacent. area herrings, 4 herring stocks appearing succesively in the region investigated. The seasonal occurrence of Anisakis larvae in herring from the examined fishing regions is related to the sequence of arrival of the different stocks. Suspectect'sources of invasion are• tliscussed, the Anisakis invasion being related to the North Sea herring migrations.

In tlte present paper the results obtained during in vitro rearing the 3rd stage larvae of Anisakis simplex (Rud.) from Baltic herring and eggs produced by the reared females are summarized. The morphology of developmental stages obtai ned, i.e., the 3rd and 4th larval, adult males and females, eggs and first larval stages, is described. Different organs being for med during metamorphosis of the nematodes reared are foll owed.

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Larvae of two nematode species: Anisakis simplex (Rud.) and Contracaecum aduncum (Rud.) were found to occur in Baltic cod ( Gadus morhua callarias L.). Their occurrence in various fish organs was analyzed: the investigations covered also the invasion incidence fish size relationship, the annual variability of infestation, and its occurrence in different re gions of the Southern Baltic.