In the previous studies on the antibiotic sensitivity of Porphyra, the somatic cells isolated enzymatically from Porphyra gametophytic blades were often used. However, the accurate analysis of antibiotics effects on the survival, growth and development of the somatic cells was difficult, because the somatic cells isolated even from a Porphyra blade had different regeneration capacity, cell division and developmental pathways when they were culturedin vitro. In this paper, the antibiotic sensitivities of Porphyra yezoensis conchospores which are almost the same in viability, germinability and developmental pathways, were studied by treatment with kanamycin, ampicillin and chloramphenicol. The obtained results demonstrated that kanamycin could promote the growth and development of conchospore germlings. After 7 days of culture, the cell number of 26% of the conchospore germlings was over 20 cells, which was twice as many as that of control group. After 11 and 18 days of culture, the mean length of conchospore germlings was 1.6 times and 2.1 times longer than that of control group, respectively. The treatment with ampicillin could inhibit the growth and development of conchospore germlings, showing decrease of cell division rates and inhibitation of germling development. After 7 days of culture, the cell number was less than 15 in 80% of conchospore germlings. After 11 and 18 days of culture, the mean length of conchospore germlings was shorter than that of control group, as ampicillin concentrations increased. The chloramphenicol had strong lethal effect on conchospore germlings. When chloramphenicol concentration was higher than 100 μg/mL, the conchospore germlings died out after 7 days of culture. The conchospore germlings treated with 50 μg/mL of chloramphenicol also died out after 11 days of culture. The cell number was less than 5 cells in 90% of the dead conchospore germlings. Therefore, it was concluded that the chloramphenicol is an effective selective pressure in genetic engineering of P. yezoensis.