Accumulations of radionuclides in marine macroalgae (seaweeds) resulting from the Fukushima 1 Nuclear Power Plant (F1NPP) accident in March 2011 have been monitored for two years using high-purity germanium detectors. Algal specimens were collected seasonally by snorkeling at Nagasaki, Iwaki, Fukushima Prefecture (Pref.), Japan, ca. 50 km perimeter from the F1NPP. Additional collections were done at Soma, Hironocho, Hisanohama and Shioyazaki in Fukushima Pref. as well as at Chiba Pref. and Hyogo Pref. as controls. In May 2011, specimens of most macroalgal species showed ¹³⁷Cs levels greater than 3,000 Bq kg⁻¹ at Shioyazaki and Nagasaki. The highest ¹³⁷Cs level recorded 7371.20 ± 173.95 Bq kg⁻¹ in Undaria pinnatifida (Harvey) Suringar on 2 May 2011, whereas seawater collected at the same time at Shioyazaki and Nagasaki measured 8.41 ± 3.21 and 9.74 ± 3.43 Bq L⁻¹, respectively. The concentration factor of marine macroalgae was estimated to be ca. 8-50, depending on taxa and considering a weight ratio of wet/dry samples of ca. 10. ¹³⁷Cs level declined remarkably during the following 5-6 months. In contrast, the ¹³⁷Cs level remained rather stable during the following 12-16 months, and maintained the range of 10-110 Bq kg⁻¹. Contamination was still detectable in many samples in March 2013, 24 months after the most significant pollution.