Introduction: The oral cavity is colonized by a variety of microorganisms, including bacteria, fungi, viruses and protozoa [1]. Early oral colonizers are related to the mode of delivery, personal relationships, and the surrounding environment. These will be responsible for subsequent colonization, which will lead to a more complex and stable ecosystem in adult life [2]. The first exposure to microorganisms in children born by vaginal delivery occurs during passage of the vaginal canal, while children born by caesarean section have their first contact with the mother’s skin and with medical equipment [3]. Streptococcus Mutans is recognized as the main bacteria etiological factor of dental caries in humans [4]. The initial acquisition of Streptococcus mutans can be influenced by several factors, such as bacterial levels in maternal saliva, low birth weight, poor oral hygiene, low socioeconomic and educational level, early eruption of primary teeth, prolonged bottle feeding and still high consumption of sucrose [5]. It is documented that children with caries in a deciduous dentition have a higher risk of developing caries in the final dentition, leading to life-long oral health consequences [6]. The aim of this study was to verify the relationship between the type of delivery (vaginal/ caesarean) and colonization of Streptococcus Mutans and establish a relationship between the colonization of Streptococcus Mutans and the early caries in children aged 5 to 7 years. Materials and methods: The Committee of Ethics of the Egas Moniz Cooperativa de Ensino superior CRL approved this study unanimously. All participants signed an informed consent. This study was carried out in the Pediatric Dentistry clinics of the Egas Moniz University Clinic. The sample consists of 40 children, between 5 and 7 years of age. Saliva samples were collected to quantify the density of Streptococcus Mutans present in the oral cavity. The CRT® bacteria test from Ivoclar Vivadent was used. The mother of each child completed an inquiry that contained socio demographic questions about pregnancy, childbirth, hygiene and eating habits of children, and the DMFT index of the children was calculated. Data were submitted to a descriptive and inferential statistical analysis, using the Chi-Square and Mann-Whitney, Fisher and Kruskal-Wallis tests in IBM SPSS Statistics 24®. Results: There was no relationship between the type of delivery and the colonization of Streptococcus Mutans. Children with high values of caries index had a higher density of bacterial colonies. (p = 0.036) Discussion and conclusions: Despite the results, it would be beneficial to carry out new studies in order to facilitate the development of strategies capable of preventing or delaying the colonization of Streptococcus mutans in children.