Results presented give evidence of the existence of quasicritical, fluidlike behavior in the isotropic phase of 4-cyano-4-pentyl-biphenyl (5CB) for frequencies ranging from the static to the ionic-dominated [low-frequency (LF)] region. Despite the boost of dielectric permittivity on lowering the frequency below 1 kHz, values of the isotropic-nematic transition discontinuity (approximately 1.1 K) and the critical exponent alpha (approximately 0.5) remain constant. It is shown that the contribution from residual ionic impurities is a linear function of temperature in the critical, prenematic fluctuation-dominated region. The validity of the fluidlike and critical behavior for LF dielectric permittivity confirmed results of a derivative analysis of the experimental data: d(epsilon)/dT proportional to (T-T*)(-alpha), originally proposed for critical mixtures. Results of a preliminary test in the isotropic phase of 4-decyl-4'-isothiocyanatobiphenyl (10BT), on approaching the smectic-E phase, may indicate a general validity of results obtained.