Abstract The modes of formation of 121Sn, 123Sn, 125Sn, 127Sn, and 128Sn from thermal-neutron induced fission of 235U have been investigated by separating indium and tin rapidly, in times as short as 4 sec after fission occurred, and measuring the amount of tin isotope formed by beta decay of indium precursors. The results are summarized in the following table. Fraction of Sn nuclide from beta decay of: In(short-lived) In(long-lived) Fraction formed independently 27-hr 121sn 0·82 +0·04 0·18±0·04 <0·14 −0.·13 40-min 123Sn 0·77 +0·06 0·23±0·06 <0·15 −0·14 129-day 123Sn <0·5 <0·2 >0·5 9·6-min 125Sn 0·65 +0·10 0·35±0·10 <0·5 −·35 9·6-day 125Sn <0·5 <0·05 >0·5 4·4-min 127Sn <0·25 2·1-hr 127Sn 0·06±0·02 59-min 128Sn <0·01 In addition it was determined that 0·55±0·15 of the short-lived (30-sec) 121In is formed by beta decay of 13-sec 121Cd. The sums of independent yield of the 123Sn isomers and of the 125Sn isomers are considerably greater than those estimated from extrapolated nuclear-charge distribution parameters. This apparent yield enhancement is attributed mainly to the closure of the 50-proton shell.