Particle preparations of parsnip yellow fleck virus (PYFV) isolates A-421 and P-121, representing the two major serotypes, were made by clarifying leal extracts with ether o r butan-1-01 and concentrating the virus particles by precipitation with polyethylene glycol and differential centrifugation. The preparations contained c. 3 I nm-diameter particles comprising two sedimenting components. Top component (T) consisted of stain-penetrable protein shells with A2,,,/Azs,, = 0.8-0.9, sedimentation coefficient (L,,) = 56 S (A-421) or 60 S (P-121), and buoyant density = 1.297 g i c n i ' . Bottom component ( B ) consisted of nucleoprotein particles, not penetrable by negative stain, with A,,,,iA2S0 = I .9, sedimentation coefficient (s!,,,,,) = 148 S (A-421) or 153 S (P-121), and buoyant density = 1.520 g/cm3 (A-421) or 1.490 g/cm.' (P-121). Yields of B component particles were up to c. 1 mg/100 g leaf tissue (both isolates); yields of T component particles were up to c. 0.6 mg (A-421) or 5.5 mg (P-121) per 100 g leaf tissue. PYFV particles were found to contain a single RNA species (rnol. wt c. 3.4 x lofi, c.9800 nucleotides), constituting 40% of the particle weight, and three polypeptide species, of mol. wt ( x 10 j) 30, 26 and 24 (A-421) or 31, 26 and 23 (P-121).