This paper reports the discovery of a syntactic dialect difference between northern and southern Middle English in the grammatical implementation of the so-called "verb-second" constraint and argues that this difference is most likely a linguistic contact effect of the Viking invasions of northern and eastern England in the eighth and ninth centuries. In the South, the Middle English V2 constraint behaves as it had in Old English; that is, as a variant of the IP-V2 type, the type found in modern Yiddish and Icelandic. In the North, however, the constraint is of the CP-V2 type, as found in modern Mainland Scandinavian and in German or Dutch. It is argued that the northern form of V2 arises as a consequence of the collapse of agreement inflection in northern Middle English, which leads to a loss of V-to-I movement, just as happened in mainland Scandinavian when agreement inflection was lost there. Once, V-to-I movement is lost, IP can no longer host the topic-verb configuration that defines the verb-second phenomenon. The collapse of agreement is further argued to result from imperfect second language learning by the Scandinavian invaders, who formed a large enough proportion of the population of the North to impose substratum effects on the northern dialect. Investigation of the available documentary evidence is shown to support dating both the collapse of endings and the appearance of CP-V2 word order to the period of the Viking invasions. It is suggested that the loss of V2 word order in English, which has made it the only Germanic language that does not respect the constraint, may have resulted from the extensive contact between northern and southern speakers that lead to the mixture of northern and southern features that characterize the language of the Midlands (and London) in the late Middle English period. 1. Introduction. As has long been known, the northern and southern dialects 1 of Middle English differed considerably in their phonology, morphology and lexicon. Many of these differences have been traced to the linguistic influence in the North of the eighth and ninth century Viking invaders who first plundered, then conquered and settled in, large territories in Northumbria, Lincolnshire and East Anglia. In this paper, we will add to the list of known differences between the dialects a † Some of the results reported in this paper were presented earlier at the International Conference on Historical Linguistics at UCLA in August 1993 and appeared in a paper published in the University of Pennsylvania Working Papers in Linguistics (Kroch and Taylor 1994).