Critical appraisal of the literature data on Mytilicola intestinalis, combined with personal experience in this field, led to formulation of answers on a series of questions re the ecology and distribution of the parasitic copepod Mytilicola intestinalis Steuer. It is confirmed that Mytilicola has been the causative agent in extensive mussel mortalities. The explosion of Mytilicola in several places on the coastlines of North Sea and Atlantic Ocean should be explained in terms of Mytilicola, being a new intruder in these waters. There is no ground for the view that a special set of ecological conditions saw to the explosive development of the autochthonous species Mytilicola intestinalis, normally existing in small numbers only. Mytilicola intestinalis has a wide ecological range in all phases of its life cycle. The number of hosts living in a given volume of water, together with the amount of flushing of that water, is the main factor governing the number of parasites per host. Since two individuals of opposite sex have to meet in the intestinal tract of one and the same host, some scattered pelagic larvae cannot easily lead to establishment of a new focus of infection. Stretches of coast devoid of mussels form an almost unsurmountable barrier against the natural spreading of this parasite. It is usually man, through his multiple activities, who should be held responsible for the invasion of mussel areas previously devoid of Mytilicola intestinalis.