Ultrastructure of the marine, bitunicate, ascomycete Julella avicenniae is presented and compared with the marine Pleospora gaudefroyi. Asci of J. avicenniae possess an ocular chamber, a thick endoascus, and a thinner ectoascus. Pseudoparaphyses are enveloped by mucilage (hyphal sheath) which stains with ruthenium red. The mucilage appears to be an extension of the pseudoparaphysis cell wall and internally these cells contain an array of vesicles. Muriform ascospores are surrounded by an exosporial sheath, an electron-dense episporium and a bilamellate mesosporium. Optimum conditions for growth are 25–30°C in 100% artificial seawater glucose- yeast extract-tryptone media, but the fungus also is able to grow at 35° and at higher salinities. The ability of the fungus to withstand extremes of environmental conditions is discussed.