Little is known about the genome of Polymyxa betae and its interactions with sugar beet, due partly to the obligate nature of the protist and the patents on Beta vulgaris sequences. The identification of an ecotype of Arabidopsis thaliana compatible with the protist would help to improve this knowledge. The infection and development of P. betae in 14 worldwide ecotypes of A. thaliana were studied. The detection of plasmodia and resting spores and the production of zoospores in the roots of A. thaliana were obtained in three bioassays, using automatic immersion systems and individual glass tubes. Detection was done using molecular detection and microscopy. Compatible interactions were established between 13 A. thaliana ecotypes of the 14 that were tested and the monosporosoric Belgian strain of P. betae, A26-41. The ecotype Cvi-0 (N1096), from the Cape Verde Islands, was the most compatible with the protist. This ecotype is also susceptible to Plasmodiophora brassicae, another plasmodiophorid. Polymyxa betae infection in A. thaliana was relatively very low compared with B. vulgaris, but every stage of the life cycle of the protist was present. The spore-forming phase was promoted at the expense of the sporangial phase, probably caused by the stress of this new environment. In addition, the protist revealed a new phenotype. This new model study will allow molecular tools available for A. thaliana to be used in order to gain a better understanding of the P. betae-plant interaction during the spore-forming phase.