Despite their widespread nature and economic impact, little is known regarding the diversity and phylogeny of diatom-infecting oomycetes. While the phylogenetic affinities of Lagenisma, affecting large centric diatoms, has recently been resolved, no member of the widespread genus Ectrogella has, so far, been investigated using molecular phylogenetics. The genus Ectrogella contains about a dozen species, which are all holocarpic. The species in the genus are diverse in terms of morphology and development, and primarily set apart from other holocarpic oomycete genera on the basis of their occurrence in unicellular or colonial algae, predominantly licmophoroid and bacillarioid diatoms. Here, we report the phylogenetic placement of two oomycete parasitoids one parasitic to Pseudo-nitzschia pungens and the other parasitic to Rhizosolenia imbricata. While both parasitoids were placed outside the crown oomycete groups represented by Saprolegniomycetes and Peronosporomycetes, they did not form a monophyletic assemblage. The Rhizosolenia parasitoid was embedded amongst marine Olpidiopsis species, while the Pseudo-nitzschia parasitoid was placed as the sister clade to all remaining oomycetes. The taxonomy of Ectrogella-like organisms and Olpidiopsis is discussed and, as a consequence of morphological differences and phylogenetic placement, two new species, Miracula helgolandica and Olpidiopsis drebesii, are introduced.