  We have measured the isotope shift of the narrow quadrupole-allowed 5 2S1/2 -
4 2D5/2 transition in 86Sr+ relative to the most abundant isotope 88Sr+. This
was accomplished using high-resolution laser spectroscopy of individual trapped
ions, and the measured shift is Delta-nu_meas^(88,86) = 570.281(4) MHz. We have
also tested a recently developed and successful method for ab-initio
calculation of isotope shifts in alkali-like atomic systems against this
measurement, and our initial result of Delta-nu_calc^(88,86) = 457(28) MHz is
also presented. To our knowledge, this is the first high precision measurement
and calculation of that isotope shift. While the measurement and the
calculation are in broad agreement, there is a clear discrepancy between them,
and we believe that the specific mass shift was underestimated in our
calculation. Our measurement provides a stringent test for further refinements
of theoretical isotope shift calculation methods for atomic systems with a
single valence electron.
