Internal-mirror 543 nm HeNe lasers are frequency-stabilized on the iod
ine hyperfine transitions. The Allen standard deviation of 1 X 10(-12)
is reached after a 10 second integration time. The hyperfine spectrum
originating from the R (12) 26-0 and R (106) 28-0 lines of the B-X sy
stem of I-127(2) has been studied. The frequency differences between h
yperfine components have been measured by beat frequency techniques, a
nd the hyperfine coupling constants are fitted for the R (106) 28-0 li
ne.