Phase diagrams of water; sodium dodecyl sulfate (SDS); and a new foam booster, alkanoyl-N-methylethanolamide (C-8, NMEA-8; C-12, NMEA-12; C-16, NMEA-16), were constructed at 25 degreesC. NMEA is hardly soluble in water: liquid-liquid phase separation occurs with NMEA-8, lamellar-phase formation occurs with NMEA-12, and solid precipitation occurs with NMEA-16. In the presence of a small amount of NMEA, the surfactant hexagonal phase (H-1) is extended to the dilute region. In the aqueous micellar solution beyond the H-1 phase, the viscosity dramatically increases and a viscoelastic solution is formed in NMEA-12 and NMEA-16 systems. The viscoelastic micellar solution formed in SDS-NMEA-12 systems follows the Maxwell model typical of wormlike micellar systems at low shear frequencies. In the SDS-NMEA-16 system, a gel-like highly viscoelastic solution is formed in the maximum-viscosity region. Rheological measurements show that the ability of NMEA to induce micellar growth increases in the following order: NMEA-8 much less than NMEA-12 < NMEA-16. In agreement with this result, dynamic light scattering measurements show that with an increasing mixing fraction of NMEA-12 or NMEA-16 in SDS-NMEA systems the micellar size increases, leading to the formation of wormlike micelles.