Despite of its success, the carrageenan industry has had to cope with difficulties due to epiphyte infestations and diseases known as ice-ice. Some promising results in respect of biomass production, carrageenan yield, and protection epiphytes were obtained using a powdered extract of the brown alga Ascophyllum nodosum in Kappaphycus alvarezii seedling production. This study focused on the effects of the A. nodosum extract on the treatment of K. alvarezii cultivated on commercial floating rafts not only to evaluate improvements demonstrated in vitro but also the effects on the quality of the carrageenan. The seedlings were treated in an A. nodosum extract solution and placed alongside their controls on commercial floating rafts using the tubular net technique. Daily growth rate, carrageenan yield, gel strength, and gel viscosity were obtained over 20 and/or 40 days. After 20 days, daily growth rates showed no significant difference (p =0.44),while the carrageenan yield was higher in samples that were treated with the A. nodosum solution (p <0.001). After 40 days, both daily growth rate (p =000.7) and carrageenan yield (p =0.009) were higher in treated samples; however, gel strength was higher in control samples (p =0.03) as viscosity was also highest in the samples which had not been treated (p < 0.001). The use of the A. nodosum extract treatment on the cultivation in situ of K. alvarezii was positive since after 40 days when the daily growth rate and carrageenan yield increased. In spite of the negative effect on the quality of the semi-refined carrageenan, the values were within commercial standards.