In eastern Austria (Lower Austria, Burgenland), three populations of the obligatory parasitic Alton blue butterfly Maculinea alcon were investigated. The finding of prepupal larvae in nests of the ant Myrmica scabrinodis is the first host ant record in Austria. Based on egg counts, the Maculinea alcon population sizes were estimated and the metapopulation structure assessed. Based on the data from the ant assemblages, the estimated population sizes of the larval foodplant Gentiana pneumonanthe and the known threats, the viability of the metapopulations was assessed. Accordingly, each of the five recent metapopulations of Maculinea alcon in eastern Austria has less than 50 individuals and must be regarded as critically endangered. We propose a protection plan including (1) safeguarding of the open and nutrient-poor character of the habitats, (2) stopping drainages, ploughing and intensification of the habitats, (3) safeguarding of the minimal area, (4) special protective measures for Gentiana pneumonanthe, (5) connecting habitat fragments, (6) reducing high grazing pressure due to row deer in small-sized habitats, and (7) establishing the ideal nest density of host ants. The extinction of Maculinea alcon in eastern Austria can be prevented only if protective measures are realised immediately.