The first spindle formation in karyogamy-blocked zygotes in the isogamous brown alga Scytosiphon lomentaria was studied by electron and immunofluorescence microscopy using anti-centrin and anti-β-tubulin antibodies. Zygotes blocked in karyogamy by incubation at high temperature (22 °C) were compared with normal zygotes incubated at 14 °C. In normal zygotes, after the disappearance of female gamete centrioles, centrioles from the male gamete duplicated and migrated to both poles in the metaphase, and a bipolar spindle was formed. The nuclear envelope was almost intact except at the poles. In karyogamy-blocked zygotes of S. lomentaria, both DNA synthesis and chromosomal condensation occurred in the male and female nuclei. A bipolar mitotic spindle with a pair of centrioles at each of two poles could be observed in the male nucleus, but not in the female nucleus. In some cases, after a pair of centrioles derived from the male gamete duplicated and separated, one of the two pairs migrated towards the female nucleus. In this case, a monopolar spindle was formed independently in each of the female and male nuclei. In addition to the monopolar spindle, a bipolar spindle was formed lying across the unfused male and female nuclei. Male and female chromosomes were intermixed and some of them were incorporated into the spindle. After cytokinesis of the normal and karyogamy-blocked zygotes, two daughter cells, each with one nucleus and one chloroplast, were formed.