Suid herpesvirus 1 (SHV-1) is the causative agent of Aujeszky's disease (AD, pseudorabies), known worldwide as a major economical threat to pig farming. Measures are taken in many countries to control or eradicate the disease and considerable effort therefore has been focused on the elucidation of the epizootiology of the infection. These studies were greatly facilitated by the possibilities, first reported in the early eighties, to identify SHV-1 strains by means of restriction fragment pattern (RFP) analysis. In the present thesis some molecular biological aspects of SHV-1 are reviewed. In addition, studies are reviewed focusing on three topics: (i) methodological aspects of molecular strain identification, (ii) various epizootiological features of the SHV-1 infection, and (iii) the population dynamics of coexisting virus particles comprising a field strain, tentatively defined as a transmissible entity. A systematization of the European isolates of SHV-1 was elaborated based on molecular characterization of various genome types and an evolutionary tree for some of the distinct types was suggested. For some of the types geographical niches could be identified indicating that the intertypic differences had been stable for decades. By the characterization of 5-10 isolates from each herd, in which a SHV-1 strain had been newly introduced, strain inherent non-intertypic genomic variations consistently could be demonstrated. Some strains appeared to consist of fairly homogeneous pools of genomic variants, while other strains appeared highly heterogeneous. Some exhibited hypervariable regions in the genome. The pool of genomic variants present in a strain was found to be a highly specific and most often a conservative characteristic of a strain. Yet, fluctuations in the proportions of subpopulations occasionally were seen. Thus, while one isolate from an outbreak might be a poor representative of the pool of variants comprising the causative strain, the analysis of 5-10 isolates from each outbreak might taken together provide the basis of an extremely fine resolving potential. Outbreaks of AD during the finishing stage of the Danish eradication campaign were subjected to intensive molecular epizootiological studies. The analysis of representative older isolates of SHV-1 revealed that only type III was present in Denmark prior to 1985. In 1985 type IIa isolates emerged in a border area, and since 1986 type IIp and IIa were the only types identified in Denmark. Severe epizootics have been recorded in border areas in Denmark since the winter of 1986/87.(ABSTRACT TRUNCATED AT 400 WORDS)