Management of rare and threatened species requires an understanding of processes limiting population establishment and growth. The Large blue butterfly Phengaris arion needs two other organisms, the Thyme plant Thymus serpyllum and the ant genus Myrmica, for the completion of its life cycle. The large blue butterfly is very rare in Sweden and also declining in Europe, and both the large blue and Myrmica are red-listed and in need of protection. Studies conducted in Great Britain have suggested that a lack of suitable habitat with the presence of the host plant thyme and Myrmica limits the distribution of the large blue. A good environment for the butterfly is thus a habitat where all three organisms can exist. To examine whether vegetation structure and abundance of Thymus serpyllum and Myrmica ants are good predictors of the presence of the large blue in Sweden, I investigated sites where the butterfly has been observed or reportedly gone extinct in the last five years. I studied one site in the region of Stockholm, five sites in the Uppsala region, and 10 sites on the island of Gotland. At each site, I recorded an abundance of thyme and its flowering status, the presence of shrubs, trees, and open vegetation, and any observations of the large blue.  Areas with the large blue tended to have a higher abundance of thyme plants and a higher percentage of thyme flowers in bloom during my visit than areas without the large blue. The difference in abundance of shrubs and trees between sites with and without the butterfly was small. Areas with the large blue also tended to have shrubs and trees compared to areas without the large blue. Because the study species are rare and endangered, the number of sites with current and past occurrence is limited, which complicates the statistical analysis of factors influencing its distribution. However, the results indicate the importance of a high abundance of the host plant for the occurrence of the large blue, but also that the host plant is much more common than the butterfly. Ants of the genus Myrmica were observed at only two of eight sites with observations of the large blue.  This suggests that the method used to detect Myrmica ants was not efficient, or that the large blue is not critically dependent on Myrmica ants in Sweden. To conclude, the remaining populations of the large blue should be monitored and measures to improve habitat quality should be considered at sites where it has gone extinct. We can try to reintroduce species in areas where the species has previously existed with strong populations of the thyme and the ant. Create a similar reintroduction as the success story of the large blue butterfly in Great Britain.