[en] Daily air monitoring of radionuclides in the Principality of Monaco (43°73′N, 7°43′E) after the Fukushima Daiichi nuclear power plant accident showed that only Iodine-131 (¹³¹I) and Caesium isotopes (¹³⁴Cs and ¹³⁷Cs) were detected. The peak of ¹³¹I varied and reached its maximum between March 29th and April 5th, meanwhile both peaks of ¹³⁴Cs and ¹³⁷Cs arrived later and attained a maximum between April 1st and 4th. Their maximum activity concentrations in air were 354, 30, and 37 μBq m⁻³ respectively. The ¹³⁴Cs to ¹³⁷Cs activity ratio was close to 1, which is different from that one observed after the Chernobyl accident (around 0.54). Up to 95% of caesium isotopes were washed out by wet scavenging during 27–28th of March, where the maximum deposition rates of ¹³⁴Cs and ¹³⁷Cs (13.7 and 19.1 mBq m⁻² day⁻¹, respectively) were observed. The significant input of ¹³⁴Cs and ¹³⁷Cs into the Mediterranean seawater column (30 m depth) was detected later, on the 24th of May. Radioisotopes of caesium and iodine were found far above the applied detection limits, but still with no concern for harmful radiation exposure and public health. The contamination gradually decreased in air and activity concentrations returned to background values after one or two months. - Highlights: ► Daily air concentrations of radionuclide in Monaco have been monitored. ► Radionuclides originating from the Fukushima accident were detected. ► Iodine-131 and ¹³⁴Cs and ¹³⁷Cs were detected in low levels, 354, 30, and 37 μBq m⁻³. ► The peak of ¹³¹I varied and reached its maximum between March 29th and April 5th.► ¹³⁴Cs and ¹³⁷Cs were later than ¹³¹I, and attained a maximum between April 1st and 4th.