The present work aimed to study the possible relationship between the Escherichia coli counts and the concentrations of free CO2, dissolved oxygen, reactive phosphorus, total ammonia, nitrite and pH in water samples of the Lagoa da Parangaba (Fortaleza, Ceará). Eight campaigns were carried out every fifteen days to collect sub-superficial water samples in three sampling spots. Regression analyses were performed upon the variable with the greatest linear Pearson coefficients looking for the curve that best fitted to the observed data. The correlation between the water free CO2 and E. coli was best represented by the equation y = 17.74 – 1.16*10-3x + 6.76*10-8x² (R² = 0.75; P = 0.06), where x is the E. coli count (MPN 100 mL-1) and y is the water carbon dioxide concentration (mg L-1). It was concluded that the rich organic matter inflows of the Lagoa da Parangaba, mainly of fecal origin, have increased not only the sanitary risk to the population that use this water source, but also affected the life of the animal populations that live in it. It was also observed that the Lagoa da Parangaba’s waters are highly contaminated by E. coli and can be classified as hypoxic and acid.