The forest pathogen Dothistroma pini (Scirrhia pini) infects the needles of many pine species, causing needle loss and consequently retarded wood growth. Only one strain of Dothistroma pini is present in New Zealand. Because over 90% of commercial forests in New Zealand are planted with the susceptible species Pinus radiata, a study of the global diversity of D. pini strains was initiated to assess the threat of further unwanted introductions of the pathogen. A collection of D. pini strains from eight countries was studied in the UK. The production of dothistromin toxin by the strains, and DNA sequence analysis of the ribosomal ITS region, confirmed their identification as D. pini, although strains from the central USA contained two nucleotide substitutions in the ITS region. Colony morphologies and growth rates were diverse, but all strains which sporulated showed a similar wide range of spore size. The morphological features examined did not support separation of the strains into the two groups shown by ITS sequences. Most striking was the production, in axenic culture, of extremely high levels of dothistromin toxin by strains from Germany and, to a lesser extent, some from the USA (>500 times and >40 times as much as the New Zealand strain, respectively). The high level of production of dothistromin toxin by some strains is a concern for forest health as well as for forest workers and needs to be evaluated further.