Phlyctis argena is lichenized with Dictyochloropsis splendida and not with D. reticulata, as thought formerly. This results from in-situ investigation of over 150 lichen thalli and more than 70 cultures of the phycobiont. Material studied from a wide range of geographical locations indicates that specificity of the association in this case is highly probable. Compared to the free–living state, the alga as a lichen partner undergoes distinct changes. Differentiation of strains of D. splendida with respect to morphology and development was not observed.