This study begins with a description of the infective process in the dinoflagellate type host Alexandrium minutum by a strain of the parasitoid, Parvilucifera sinerae, including the morphologies of the various dinoflagellate and parasitoid stages during the infection. Then, the susceptibility of 433 microalgal strains to P. sinerae infection was studied. The parasitoid was found to be capable of infecting several dinoflagellate species of the genera Alexandrium, Coolia, Dinophysis, Fragilidium, Gambierdiscus, Gymnodinium, Gyrodinium, Heterocapsa, Kryptoperidinium, Lepidodinium, Ostreopsis, Pentapharsodinium, Protoceratium, Scrippsiella, and Woloszynskia. Intra-strain variability was observed as well, such that within the same dinoflagellate species some strains were infected whereas others were not. Likewise, species of other dinoflagellate genera were not infected, such as Akashiwo, Amphidinium, Barrufeta, Bysmatrum, Karenia, Karlodinium, Prorocentrum, and Takayama. Moreover, P. sinerae was not able to infect any of the tested haptophyte, diatom, and chlorophyte species. In natural samples screened for P. sinerae infectivity, several dinoflagellate species of the genera Alexandrium, Coolia, Gonyaulax, Gymnodinium, Phalacroma, Protoperidinium, and Scrippsiella were identified as susceptible. Sporangia size was found to be proportional to the size of the host, and variations in the sporangia size were observed to influence their maturation time.