Results of a detailed study on the vertical distribution of fouling and wood-boring organisms of Trondheimsfjord at an interval of 3 m from intertidal level to a depth of 30 m, has been presented, based on data collected from two series of panels, exposed from 15 March 1977 to 15 July 1977 (Series I) and from 22 July 1977 to 13 March 1978 (Series II). The intensity of fouling generally decreased with increasing depth. Quantitatively, fouling was heavy on panels of Series I more than on Series II, although species-wise it was more heterogenous on panels of Series II. The bulk of the fouling was constituted by Balanus crenatus, Laomedia sp., Mytilus sp., Modiolus sp. and Hiatella arctica. Incidence of borers and the resultant destruction of timber were heavier on panels of Series II than on those of Series I. The influence of the period of exposure on the above pattern of infestation by foulers and borers has been discussed. Psiloteredo megotara concentrated at the upper levels up to a depth of 15 m, with more settlement between 3 m to 9 m depth. Although Xylophaga dorsalis was present on panels from 3 m to 30 m depth, their intensity abruptly changed from 9 m onwards and continued to increase with increasing depth, with a maximum number near the mud level at 30 m. Attack of Limnoria lignorum also was heavy at the mud level. On the same panel, while Psiloteredo megotara preferred to settle in more numbers on the lower surface, Xylophaga dorsalis did so on the upper silted surface. The importance of such selective vertical incidence of different borers and their co-operation in the destructive activity have been stressed from the point of wood destruction in the Tronheimsfjord. The rate of growth of Balanus crenatus, Laomedia sp., Psiloteredo megotara, Xylophaga dorsalis, and Xylophaga praestans in relation to depth has been presented. For Balanus crenatus rate of growth decreased with increasing depth, while for Laomedia the same increas with depth up to 12 m and declined thereafter. In accordance with its depth preference, Psiloteredo megotara registered faster growth between 3 to 15 m depth. In the case of Xylophaga dorsalis, the size of the shell valve and burrow increased with increasing depth up to 24 to 27 m and then showed a slight decline at 30 m. Factors influencing the growth-rate at different levels have been discussed. The results on the vertical zonation and rate of growth of the wood-infesting organisms encountered, have been compared with relevant literature published earlier.

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